Before this change `xfce.garcon` did not always compile successfully:
$ c=1; while nix build --rebuild -f. xfce.garcon --cores 1; do (( c+=1 )); echo again with $c; done
...
again with 4
garcon-gtk/Makefile.am: installing './depcomp'
.../am/depend2.am: error: am__fastdepCC does not appear in AM_CONDITIONAL
.../am/depend2.am: The usual way to define 'am__fastdepCC' is to add 'AC_PROG_CC'
This happens due to an autoconf bug when m4 is called within 1 second:
Upstream report and fix: https://savannah.gnu.org/support/index.php?110521
The change allows `xfce.garcon` to rebuild 60 times in a row without failures.
