Merge pull request #3096 from gkhanna79/MonoCheck

Check for Mono presence until CoreCLR 3090 is fixed.

Author: gkhanna79

build.sh

@@ -222,6 +222,8 @@ build_mscorlib()
        return
     fi
 
+    hash mono 2> /dev/null || { echo >&2 "Skipping mscorlib.dll build since Mono is not installed."; __SkipMSCorLib=1; return; }
+
     # Restore buildTools
 
     restoreBuildTools