Use "Subversion_SVN_EXECUTABLE" explicitly instead of "svn patch".

Even though"Subversion_SVN_EXECUTABLE" is set automatically when
configuring the build, running the "googletest" .csproj fails due to
supposedly missing svn command.
Correction to commit e7ab4f6
Merge pull request #161 from easuter/devel

Use "Subversion_SVN_EXECUTABLE" explicitly instead of "svn patch".