Both GLib and GStreamer have a config file called config.h and use the HAVE_CONFIG_H macro to conditionally include them.
Normally, this works out fine, but if they are subprojects, we may accidentally trigger including a different config.h. This is normally harmless, but it causes a bunch of warnings because we define GETTEXT_PACKAGE on the command-line, and it won't match the one in a different project.
However, we only include this file in source files, not headers, and it's always created, so there's no need to conditionally include it or define the macro. We can also rename it so it can't clash with other instances as well.

Compiled with subprojects and stopped seeing many warnings about GETTEXT_PACKAGE and other generic variables being re-defined due to GStreamer's config.h.