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
D-Bus improvements

* Implemented SetPosition in D-Bus, this allows external programs to jump to a specific position
* Fixed Seek so that it obeys the passed relative seek value, as per MPRIS
* Added Position and Duration properties which return the current position and media duration in microseconds
* fixed some overflow errors by using int64_t instead of long, position was wrapping at around 31 minutes
* A little extra debug logging for D-Bus method calls, useful when trying to determine why it's not responding to a method
* Added a sample D-Bus control shell script
* Explicitly specify KeyConfig enum constants, in case external programs depend on them
* Changed D-Bus session handing so a single dbus-daemon is started the first time omxplayer starts, and continues running forever
* Added argument --dbus_name to override the default org.mpris.MediaPlayer2.omxplayer
Merge pull request #146 from blakee/dbusfixes

Single-Session D-Bus / D-Bus control fixes