gst1-plugins-bad: fix egl detection

Some egl implementations needs special compiler flags or defines (E.G.
mesa3d for wayland needs to pass -DMESA_EGL_NO_X11_HEADERS to not try to
include X11 headers).  This is correctly included in the .pc file, but the
configure logic doesn't use pkg-config so help it along.

Likewise, some implementations may need to link against other libraries, so
also pass LIBS.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>