This was modified slightly from Gnulib. In Gnulib, it expects the
@HAVE_SYS_CDEFS_H@ to be replaced. Instead, we can set HAVE_SYS_CDEFS_H
on systems that have it and avoid copying another file into the build
directory. Since we are not using gnulib-tool, copying extra files
requires extra build system updates (and special handling with CMake) so
we should avoid when possible.
