docs/autoconf: update to better reflect reality

docs/autoconf: update to better reflect reality

 * --enable-{32,64}-bit is done. Use --build and --host instead.
 * Configure does not add "-g -O2" to C{,XX}FLAGS.
 * Pkg-config has been mandatory for a while now.
 * Avoid using LDFLAGS, refer to pkg-config.
 * --with-expat is deprecated. Use pkg-config.

v2:
 * Note that CC/CXX will need to be set for multilib builds.
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Matt Turner <mattst88@gmail.com> (v1)