Cygwin port

Matching pull request to https://github.com/Andersbakken/rct/pull/3 that adds support for cygwin

One big cmake issue I ran into was that changing rct sources and rebuilding (running make) would not incorporate the changes into rdm.  I rearranged them a bit to correct this (for my machines at least), but you may find these changes inappropriate.
  
I've verified the following basic functionality under both Linux and Cygwin:
- project listings
- finding symbols / references
- loading compilation databases
- compiling single files under a project

Please let me know if it's useful [or not] - comments, flames, issues, etc.  Thanks for rtags!