The AM_CONDITIONAL always need to be evaluated, regardless of
whether we are building with or without gl plugins (the actual
checks are only called in AG_GST_GL_PLUGIN_CHECKS).
If libjpeg is not available, leave HAVE_JPEG undefined in
config.h, instead of defining it to 0. Fixes mismatch between
autotools conditional and ifdefs in the code.
Tests and documentation will follow separately.
The mixer elements in the opengl plugin need to stay
in -bad for now since they use GstVideoAggregator.
https://bugzilla.gnome.org/show_bug.cgi?id=754094
