windowx11glx: use fbconfigs if supported
feature: make gst_gl_check_extension public

prevents duplication of code of extension string parsing in GstGLWindow's
window_x11: add XError trapping

some functions in GLX can fail if the implementation does not support
certain features we ask for (i.e. GL 3) bringing down the entire
application
x11_glx: ask for a 3.0 GL Context

(falling back if needed)
build: add support for static plugins
build: remove libtool warning about -version-info not used in convenience libraries