The GBM library detection does not need to be in the configure script,
since it does not have any user-facing options (there are no
--enable-gbm or --disable-gbm switches). Let's move it to meson.build
instead, so we don't have to clutter config-host.mak with the related
switches.

Additionally, only check for GBM if it is really required, i.e. if we
either compile with OpenGL or with virglrenderer support.

Message-Id: <20210714085045.797168-1-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>