Detect and propagate _FORTIFY_SOURCE prefix

Get prefix to a -D_FORTIFY_SOURCE string if it is present in
CFLAGS and apply the prefix to flags added to redefine
_FORTIFY_SOURCE in CFLAGS and CPPFLAGS

* fixes 1569
Merge pull request #1654 from gaurdro/rpm-fortify_source

[RFC] Fix issue 1569: Build fails in rpmbuild environment