I needed to patch this to package wlopm for chimera linux, which uses
FreeBSD's coreutils. Apparently -D is not portable, it means something
different for FreeBSD install.
The makefile already use (directly, or indirectly) CPPFLAGS and
LDFLAGS, so it makes sense to also use the user provided CFLAGS.
One example where this is necessary is if the user has set
CPPFLAGS=”-D_FORTIFY_SOURCE=2”, which, before this patch, results
in (even if the user has set CFLAGS=-O2):
/usr/include/features.h:397:4: error: #warning _FORTIFY_SOURCE requires compiling with optimization (-O) -Werror=cpp]
