Revision 123271

speed up the 'perl Makefile.PL' stage
 - in ModPerl::BuildMM, introduce an overall $build, and call
   build_config() to define it only when it's not defined
 - in Apache::Build, change the apxs_cflags, apxs_extra_cflags,
   and apxs_extra_cppflags subs to call apxs as a method, if
   possible, so as to cache the result of the call
 - in the apxs sub of Apache::Build, return the cached result
   only if it's defined