gplugin/gplugin

Make sure gtk4 was found before trying to build vala bindings against it

This error only showed up when gtk4 was enabled but not found.

Testing Done:
Built on haiku that doesn't have gtk without setting `-Dgtk4=false`.
Built on debian testing normally.

Reviewed at https://reviews.imfreedom.org/r/3069/
e = executable('test-python3-loader', 'test-python3-loader.c',
include_directories : include_directories('.'),
c_args : [
'-DPYTHON3_LOADER_DIR="@0@/.."'.format(meson.current_build_dir()),
'-DPYTHON3_PLUGIN_DIR="@0@/plugins"'.format(meson.current_source_dir()),
],
link_with : gplugin_loader_tests,
dependencies : [GLIB, GOBJECT, PYTHON3, PYGOBJECT, gplugin_dep])
test('Python3 loader', e,
depends: gplugin_python3)
# we can't see the symbols in gplugin-python3 externally, so use the static
# version for testing
e = executable('test-python3-utils', 'test-python3-utils.c',
include_directories : include_directories('.'),
link_with : [gplugin_loader_tests],
dependencies : [GLIB, GOBJECT, PYTHON3, PYGOBJECT,
gplugin_python3_static_dep])
test('Python3 utils', e)
# Add the test plugins path to our environment variable
devenv.append('GPLUGIN_PLUGIN_PATH', meson.current_source_dir() / 'plugins')