libX11 1.7.3 tarball is missing ax_gcc_builtin.m4
cc @alanc
Presumably an incomplete autotools run before make dist?
./configure: line 21761: syntax error near unexpected token `__builtin_popcountl'
./configure: line 21761: `AX_GCC_BUILTIN(__builtin_popcountl)'
Reproduced on Fedora 34 and current Rawhide