build: Do not mess with CFLAGS in configure.

* configure.ac: Do not mess with the user provided CFLAGS.

A problem was claimed with some configure tests if the user provided
CFLAGS=-Werror. The commit introducing this

does not mention a concrete case. Anyway, messing with CFLAGS is a
bad idea because configure tests will then test something different
than what is used later (cf. autoconf manual). Tests which depend on
the whether -Werror is used needsto be fixed.

Note that in certain cases we modify CFLAGS. This is only done for
some configure options or if the platform requires the use of special
compiler flags (e.g. on HP/UX).

• Fixes-commit: 02eb9fc9d5863abcfed6af704e618f8cac7cc2e8
• GnuPG-bug-id: T2423