WinCE: Fixing errno.h to define a replacement errno.
WinCE: Remove .gitignore entries which are no longer needed.
WinCE: Changing code to use _WIN32_WCE as preprocessor define indicating WinCE.
Core: Detect <signal.h> and <sys/types.h> using configure or platform config.h.

Also update msvc/config.h to define appropriate defines for these headers.
WinCE: Adding documentation on how to build for Windows CE.