Simplify configure.ac and Makefile.am logic.
* Add support to specify the gstreamer version to the configure script as parameter (--with-gstreamer-version=1.0). If the parameter is not specified, we automatically check for the latest gstreamer version (1.0), if not found, we try the old version (0.10).
* Use only one variable to specify the cflags and one for the libs. Before we passed one variable for gstreamer base, one for gstreamer, etc.
* Rename define HAVE_GSTREAMER_1_0 to GSTREAMER_1_X.