improve the build process under native pi

$(MAKE) will always be set to the make thats running this makefile, so if it happens to be named differently, it still works
+ allows the job server, so if the user happens to make -j10 while using distcc on an array of pi's, it works right
-C ffmpeg does the same thing as cd ffmpeg

change the CC and CXX variables to reference gcc 4.7 directly, rather then forcibly change the symlinks
add the dbus compile flags and install the dbus libs
Merge pull request #145 from cleverca22/incoming

 improve the build process under native pi