[flac-dev] Updated CFLAGS patches and make test compilation conditional

[David Seifert]

Hi Erik,
I've found a middleground for the problem of setting default CFLAGS. I've gone back
to setting them if {C,CXX,CPP,LD}FLAGS are unset at the onset of the configure script
(i.e., the user hasn't specified anything) and then proceed to set them to the defaults
as before. This has been suggested before:

  https://lists.gnu.org/archive/html/autoconf/2006-04/msg00022.html

In addition, I've added a small compile snippet to check whether _FORTIFY_SOURCE is
defined, and if so, do not add it to CPPFLAGS. This is mainly due to the fact that
Gentoo's GCC has _FORTIFY_SOURCE always enabled, which causes a ton of superfluous
redefinition warnings when adding this flag to CPPFLAGS.

Finally, I forgot to add one patch from our repo, namely that noinst_ is the wrong
prefix for test programs. These should be prefixed with check_, which causes them only
to be compiled when 'make check' is run, making the overall build faster.

Regards
David