nscd: fix compilation flags

Commit 61653dfb added support for
compilers predefining _FORTIFY_SOURCE by adding -U_FORTIFY_SOURCE to
CPPFLAGS for these compilers, but that change doesn't work quite well in
case of nscd: its Makefile sets _FORTIFY_SOURCE using CFLAGS instead of
CPPFLAGS and, thanks to compilation rules defined in Makerules, CPPFLAGS
are passed to compiler after CFLAGS, resulting to a build with
_FORTIFY_SOURCE turned off.

This change implements a more safe method of passing preprocessor and
compiler flags so that no nscd modules could be accidentally forgotten.