configure: Test for _FORTIFY_SOURCE=3 support.

_FORTIFY_SOURCE=3 adds extra glibc (dynamic) fortification checks
when using GCC 12.

This adds a configure check to see if -D_FORTIFY_SOURCE=3 can be used.
If not, configure will fall back to -D_FORTIFY_SOURCE=2.

On some older glibc versions (glibc 2.17) using -D_FORTIFY_SOURCE=3
provides the same fortification as _FORTIFY_SOURCE=2. On some newer
glibc versions and older GCC (glibc 2.34 amd gcc 11) using
-D_FORTIFY_SOURCE=3 produces a not supported warning (and we fall
back to -D_FORTIFY_SOURCE=2). With newer glibc and newer GCC versions
(glibc 2.35 and gcc 12) -D_FORTIFY_SOURCE=3 will use the newer dynamic
fortification checks.

This patch also makes sure that AC_PROG_CXX is used earlier so that
CXXFLAGS is always setup correctly (even if we then don't use it).
And it outputs both the CFLAGS and CXXFLAGS as used at the end.

Signed-off-by: Mark Wielaard <mark@klomp.org>

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