Meson: build groups even without gap

`group.pyx` compiles fine also without gap installed.

src/sage/groups/meson.build

@@ -38,13 +38,17 @@ extension_data = {
 }
 
 foreach name, pyx : extension_data
+  deps = [py_dep, gmp]
+  if name == 'libgap_wrapper'
+    deps += [gap]
+  endif
   py.extension_module(
     name,
     sources: pyx,
     subdir: 'sage/groups',
     install: true,
     include_directories: [inc_cpython],
-    dependencies: [py_dep, gmp, gap],
+    dependencies: deps,
   )
 endforeach