build: Remove unneeded OS detection section

Booth is not using any of detected values and it is also not linking any
shared library so extra compiler flags are not needed too.

Instead use standard AC_USE_SYSTEM_EXTENSIONS which defines required
macros. Also make sure b_config.h is always included as a first header so
defined system extensions have effect.

Signed-off-by: Jan Friesse <jfriesse@redhat.com>