These subdirs have unique configure scripts to do some compiler tests.
The checks should work for all targets, so hoist them up to the top
libgloss dir. This should allow us to delete these subdir configure
scripts.
It means the top-level gains autoheader support, but that's fine.
It wasn't exporting any defines previously (i.e. -D into CPPFLAGS),
and all of the defines it now exports are only used by code in the
libnosys subdir which was expecting to have a config.h.
