This commit makes the loading of the GModules threadsafe, and
always first tries to load the symbol for the GL library that
is selected for the current context. Only then it falls back
to looking into the current module (NULL), and only as a last
resort the context specific function (e.g. eglGetProcAddress())
is called.

Also add configure parameters to select the names of the library
modules instead of using the defaults, and let the defaults be
independent of the G_MODULE_SUFFIX.

https://bugzilla.gnome.org/show_bug.cgi?id=728753