Filter out -no-cpp-precomp from flags passed to SWIG.

This is necessary since APR for whatever reason leaks the fact that it uses
-no-cpp-precomp on OS X into apr-1-config.  Unfortunately, a lot of versions
of APR have this in the wild so we just have to deal with it.  If you use clang
directly you don't see this because we already filter it out of CPPFLAGS.

* Makefile.in
  (SWIG_CPPFLAGS): New variable, deliberately pulling in EXTRA_CPPFLAGS and
    not EXTRA_SIWG_CPPFLAGS because it would be harmful to split those
    (e.g. users wanting to enable a feature that adds an API).

* build.conf
  (swig-python-opts, swig-perl-opts, swig-ruby-opts): Use SWIG_CPPFLAGS
    instead of CPPFLAGS.

* configure.acc
  (SWIG_CPPFLAGS): Add the variable and copy it from the normal CPPFLAGS
    while filtering out the -no-cpp-precomp.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1535610 13f79535-47bb-0310-9956-ffa450edef68