Fixed problem with --xplat build when mono does not installed.

Author: dse

setup.py

@@ -280,7 +280,13 @@ def _get_manifest(self, build_dir):
         return manifest
 
     def _build_monoclr(self):
-        mono_libs = _check_output("pkg-config --libs mono-2", shell=True)
+        try:
+            mono_libs = _check_output("pkg-config --libs mono-2", shell=True)
+        except:
+            if DEVTOOLS == "dotnet":
+                print("Skipping building monoclr module...")
+                return
+            raise
         mono_cflags = _check_output("pkg-config --cflags mono-2", shell=True)
         glib_libs = _check_output("pkg-config --libs glib-2.0", shell=True)
         glib_cflags = _check_output("pkg-config --cflags glib-2.0", shell=True)