* .gitignore: add autogenerated file to ignore list
    * Makefile.am: conditionally add gstreamermm-plugins-bad.h file to
      installed headers list.
    * build/gstreamermm.m4: share info about enabled modules with
      preprocessor.
    * configure.ac: gstreamermm-plugins-bad.h as config header file.
    * gst-plugins-bad/gstreamermm/plugins-bad.h.in: add input header file
      for gstreamermm-plugins-bad module.