Currently there are no uses of the uppercase version when using
bsd.compat.mk with WANT/NEED_COMPAT, only within the libraries build
done by Makefile.libcompat where you instead use COMPAT_LIBCOMPAT.
However, we did used to provide this (although perhaps not as an
official interface) prior to 91d7edd549f5 ("Generalise libcompat to be a
list rather than a single option"), so let's bring it back for
completeness for any future uses tha may arise.
Note that it's called _LIBCOMPAT rather than LIBCOMPAT so as to not
conflict with the src.libnames.mk variable for the library called
libcompat.