FIX #1271: create manifests, and embed them in executables on Windows

We have 4 new flags:

  -fno-gen-manifest
     suppresses creation of foo.exe.manifest

  -fno-embed-manifest
     suppresses embedding of the manifest in the executable

  -pgmwindres
     specify a program to use instead of windres

  -optwindres
     extra options to pass to windres

"windres" is now copied from mingw and included in a binary
distribution.