Workaround issue with defining a preprocessor symbol to a string value

with embedded blanks.

The extra, escaped quoting in the original version is required for passing 
the definition over the command-line, but it broke Visual Studio builds.

For now, avoid the extra quoting when generating for Visual Studio.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1543149 13f79535-47bb-0310-9956-ffa450edef68