The conda-forge build uses xraylib-4.1.3 and seems to build OK at least. Right now the version here is something between 3.3.0 and 4.0.0 (and the version here seems to have the xraylib 4 error handling API).

When doing build testing it a git tree, it is convenient to fully clean out the tree with git clean -fdx. The problem is that because the unpacked fgsl and xraylib tarballs have .git directories, the fgsl and xraylib directories are left untouched. So it would be good of those directories were removed from the distributed tarballs.

Right now you have 2.5 and 1.4.0. I see no reason not to upgrade, these have been used for the Conda build. As far as I can tell gsl-2.7 also works with fgsl 1.5.0 (I've successfully built that way), but fgsl doesn't advertise that compatibility, though that may very well be because development of fgsl has been abandoned.

You get a bit of chatter on a Linux system when you unpack the fgsl tarball:

tar tvf fgsl.tar.gz
drwxr-xr-x dcs16/staff       0 2023-09-20 23:10 fgsl/
-rwxr-xr-x dcs16/staff     176 2023-09-20 23:10 fgsl/._acc_build
tar: Ignoring unknown extended header keyword 'LIBARCHIVE.xattr.com.apple.quarantine'
-rwxr-xr-x dcs16/staff     292 2023-09-20 23:10 fgsl/acc_build
drwxr-xr-x dcs16/staff       0 2023-09-20 23:10 fgsl/interface/
-rw-r--r-- dcs16/staff    3421 2023-09-20 23:10 fgsl/configure.ac
-rwxr-xr-x dcs16/staff     176 2023-09-20 23:10 fgsl/._acc_build_fgsl
tar: Ignoring unknown extended header keyword 'LIBARCHIVE.xattr.com.apple.quarantine'

Note the files beginning with ._. This is Mac tar adding resource forks I believe. The --no-mac-metadata flag to tar should prevent the creation of these.