Clean up rules.mk and add support for building C++ source files.

If your module uses C++, you need to add APACHE_REQUIRE_CXX to your
module's config.m4.

Unfortunately, we cannot use any sophisticated pattern matching in make,
so instead of adding the C++ source files to LTLIBRARY_SOURCES, you need
to write for foo.cpp and bar.cpp:

LTLIBRARY_OBJECTS_X = foo.lo bar.lo


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@85873 13f79535-47bb-0310-9956-ffa450edef68