Esteemed Colleagues:
I solved my problem by renaming the pagecurl plugin (removing its execute permissions, as recommended by the previous poster, also works). And the problem was definitely coming from pagecurl; here is the full text of the (former) error message, including the last line which I failed to quote in my original posting:
(gimp:16098): GLib-GObject-CRITICAL **: 15:27:51.353: g_param_spec_internal: assertion 'g_param_spec_is_valid_name (name)' failed
/gnu/bin/gimp: fatal error: Segmentation fault
(pagecurl:16112): LibGimpBase-WARNING **: 15:27:52.850: pagecurl: gimp_wire_read(): error
Although I really should not say that I solved the problem, only that I worked around it, inasmuch as I am now living without the pagecurl plugin, and therefore, presumably, cannot curl my pages. It is, perhaps, for the best, then, that I do not know what it means to curl pages, and, therefore, do not know what I am missing, and cannot have.
I still had to do one more thing to get a gimp with the functionality I needed, unrelated to the segfault problem. At first, I did not have access to any fonts. Ironically, the reason that I underwent the nightmare of compiling gimp 2.10 from source, was that the gimp2.8 provided by package manager also did not have access to any fonts. I despaired of diagnosing the problem and instead underwent the nightmare of compiling gimp 2.10 from source, which segfaulted upon startup, and then, after I worked around that problem, did not enable me to access any fonts.
I solved the font problem by renaming (removing execute permission did not, in this case, work) my local copy of the fontconfig library, which I had placed in /usr/local/lib/libfontconfig.so.1. Once that file -- which was shadowing the native /usr/lib64/libfontconfig.so.1 -- was renamed, both the native gimp2.8 and my newly-built gimp2.10 were able to access fonts. Of course, I would never have spent so much time getting the 2.10 build to work, if I had known that even after I built it, I still wouldn't be able to access any fonts.
It is curious that my local copy of libfontconfig had rendered all my fonts inaccessible, because I had not customized libfontconfig in any way. I obtained the fontconfig-2.13.92 tarball and built the library with "./configure" and "make", without any customizations (parenthetical question: why does "make install" create libfontconfig.so.1 when the version number in the tarball name is 2 and not 1?). And yet, the moment I renamed it, and allowed both gimp2.8 and gimp2.10 to dynamically link with the version in /usr/lib64, all my fonts were back.
If there is a way to mark a thread as "solved", then you have my permission to mark this thread as solved; but I will not do so myself, because I do not consider that depriving myself of the pagecurl plugin can be properly called a solution, even though I do not know what I am missing. There should be a way to build a pagecurl plugin that does not segfault upon startup. The pagecurl plugin that is part of the native gimp2.8 does not segfault (although transplanting that plugin into gimp2.10 didn't work; the transplanted plugin segfaulted in the exact same way as the one that I had built as part of gimp2.10).
I solved my problem by renaming the pagecurl plugin (removing its execute permissions, as recommended by the previous poster, also works). And the problem was definitely coming from pagecurl; here is the full text of the (former) error message, including the last line which I failed to quote in my original posting:
(gimp:16098): GLib-GObject-CRITICAL **: 15:27:51.353: g_param_spec_internal: assertion 'g_param_spec_is_valid_name (name)' failed
/gnu/bin/gimp: fatal error: Segmentation fault
(pagecurl:16112): LibGimpBase-WARNING **: 15:27:52.850: pagecurl: gimp_wire_read(): error
Although I really should not say that I solved the problem, only that I worked around it, inasmuch as I am now living without the pagecurl plugin, and therefore, presumably, cannot curl my pages. It is, perhaps, for the best, then, that I do not know what it means to curl pages, and, therefore, do not know what I am missing, and cannot have.
I still had to do one more thing to get a gimp with the functionality I needed, unrelated to the segfault problem. At first, I did not have access to any fonts. Ironically, the reason that I underwent the nightmare of compiling gimp 2.10 from source, was that the gimp2.8 provided by package manager also did not have access to any fonts. I despaired of diagnosing the problem and instead underwent the nightmare of compiling gimp 2.10 from source, which segfaulted upon startup, and then, after I worked around that problem, did not enable me to access any fonts.
I solved the font problem by renaming (removing execute permission did not, in this case, work) my local copy of the fontconfig library, which I had placed in /usr/local/lib/libfontconfig.so.1. Once that file -- which was shadowing the native /usr/lib64/libfontconfig.so.1 -- was renamed, both the native gimp2.8 and my newly-built gimp2.10 were able to access fonts. Of course, I would never have spent so much time getting the 2.10 build to work, if I had known that even after I built it, I still wouldn't be able to access any fonts.
It is curious that my local copy of libfontconfig had rendered all my fonts inaccessible, because I had not customized libfontconfig in any way. I obtained the fontconfig-2.13.92 tarball and built the library with "./configure" and "make", without any customizations (parenthetical question: why does "make install" create libfontconfig.so.1 when the version number in the tarball name is 2 and not 1?). And yet, the moment I renamed it, and allowed both gimp2.8 and gimp2.10 to dynamically link with the version in /usr/lib64, all my fonts were back.
If there is a way to mark a thread as "solved", then you have my permission to mark this thread as solved; but I will not do so myself, because I do not consider that depriving myself of the pagecurl plugin can be properly called a solution, even though I do not know what I am missing. There should be a way to build a pagecurl plugin that does not segfault upon startup. The pagecurl plugin that is part of the native gimp2.8 does not segfault (although transplanting that plugin into gimp2.10 didn't work; the transplanted plugin segfaulted in the exact same way as the one that I had built as part of gimp2.10).