Split up configure thusly:

 - build/autoconf/configure.ac: main skel
 - build/autoconf/lustre-build.m4: non-arch macros
 - build/autoconf/lustre-build-linux.m4: linux-specific macros
 - build/autoconf/lustre-build-darwin.m4: darwin-specific macros
 - portals/autoconf/lustre-portals.m4: portals checks
 - lustre/autoconf/lustre-core.m4: lustre checks
 - lustre/autoconf/lustre-version.ac: the canonical lustre version