From commit bf8bba61b8, (git: correct ac_cv_fread_reads_directories on
musl, 2018-11-14), we've correctly set ac_cv_fread_reads_directories for
git linked with musl-libc variant.
Unfortunately, glibc also allows fopen on directory. But, I was obsessed
with musl-libc at the time, hence forgot to run the test for glibc.
Correct it.
84a254b38e