The intent is to look for platform-specific facilities for regex
matching, to provide portable MENUCOLORS configuration files.
This is a prototype implementation being committed to see if Windows can
use the POSIX regex implementation provided with the C++11 standard
library. If this works, I will write a harness for POSIX regexes and for
pmatch(), and those can be linked in by platforms as appropriate.
pmatch() should be used only as a very last resort, because it breaks
compatibility between platforms.
