Skip build of testegl on RPI if no brcmegl dependency available

I want to build gst-omx using gst-build for RPi4 due to need for omxhdmiaudiosink element.

By default build of gst-omx implies build of testegl example. The example should be linked to brcmegl library for RPi target, which is deprecated and not properly functioning on RPi4.

Moreover providing pkg-config path to RPi-specific libs in order to compile testegl example causes several build errors in gst-plugins-base.

Workaround for the case should be disabling building of examples. But due to bug in in meson it's not possible to disable examples only for gst-omx, only for all the GStreamer subporjects.

It's suggested to skip build of testegl in case no RPi-specific libs found.