configure.ac: Respect LLVM_CFLAGS in LLVM version detection

configure.ac: Respect LLVM_CFLAGS in LLVM version detection

When compiling LLVM headers, including llvm-config.h, we need to respect
LLVM_CFLAGS. This is especially crucial if LLVM is located in a
non-standard location and it happens that llvm-config.h includes another
header. In such case the detection would fail due to missing header,
because the path is provided in LLVM_CFLAGS.

Let's add LLVM_CFLAGS to global CFLAGS for the time of detection and then
restore the original flags, as done in other places of the script.

Signed-off-by: Tomasz Figa <tfiga@chromium.org>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>