Only build frontend and look for GTK if its requested

Signed-off-by: Ikey Doherty <ikey@solus-project.com>

src/frontend/meson.build

@@ -0,0 +1,3 @@
+if get_option('with-frontend') == true
+    dep_gtk3 = dependency('gtk+-3.0', version: '>= 3.4.0')
+endif

src/meson.build

@@ -16,5 +16,6 @@ nica_includes = [
     config_h_dir,
 ]
 
+subdir('frontend')
 subdir('intercept')
 subdir('shim')