subversion: make python bindings build on 10.9.

Need to filter out CPPFLAGS that confuse swig, see #23993.

Subversion’s build system is complicated. build.conf is processed by a python module called by another python script which generates an auto-included Makefile that passes `$(CPPFLAGS)` to swig, but swig doesn’t know what to do with `-F/usr/local/Frameworks` or `-isystem/usr/local...`. This adds some [GNU make magic][1] to strip out those extra arguments from the `$(CPPFLAGS)` passed to swig.

[1]: https://www.gnu.org/software/make/manual/html_node/Text-Functions.html