gplugin/gplugin

Fix some issues with the gtk3 and gtk4 includes when not used in a subproject

Testing Done:
Ran the unit tests and gplugin-gtk4-viewer and verified no issues.

Bugs closed: GPLUGIN-145

Reviewed at https://reviews.imfreedom.org/r/1469/
if get_option('lua')
e = executable('test-lua-loader', 'test-lua-loader.c',
c_args : [
'-DLUA_LOADER_DIR="@0@/.."'.format(meson.current_build_dir()),
'-DLUA_PLUGIN_DIR="@0@/plugins"'.format(
meson.current_source_dir()),
],
link_with : gplugin_loader_tests,
dependencies : [GLIB, GOBJECT, LUA, gplugin_dep])
test('Lua Loader', e)
# Add the test plugins path to our environment variable
devenv.append('GPLUGIN_PLUGIN_PATH', meson.current_source_dir() / 'plugins')
endif # lua