configure.ac: set LLVM_{C, CXX, LD}FLAGS only as needed

configure.ac: set LLVM_{C, CXX, LD}FLAGS only as needed

Earlier refactoring commits started setting the above regardless if LLVM
is used or not. Move them to the respective section to restore the
original functionality.

Since we require the preprocessor flags (includes in particular) for the
header version parsing keep those as-is. They are not used outside of
configure.ac thus should not cause any side-effects.

As-is adding the C/CXXFLAGS can lead to build issues on when
cross-compiling.

Cc: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: Tomasz Figa <tfiga@chromium.org>
Cc: "17.0" <mesa-stable@lists.freedesktop.org>
Reported-by: Ilia Mirkin <imirkin@alum.mit.edu>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Tobias Droste <tdroste@gmx.de>