[build] Fix invalid escape sequence

Author: lpinca

binding.gyp

@@ -11,7 +11,7 @@
         ["OS=='mac'", {
           'variables': {
             'clang_version':
-              '<!(cc -v 2>&1 | perl -ne \'print $1 if /clang version ([0-9]+(\.[0-9]+){2,})/\')'
+              '<!(cc -v 2>&1 | perl -ne \'print $1 if /clang version ([0-9]+(\\.[0-9]+){2,})/\')'
           },
           'xcode_settings': {
             'MACOSX_DEPLOYMENT_TARGET': '10.7'