Improve toolchain detection for WinCE cross compilation:

When cross compiling WinCE with the arm-wince-cegcc-gcc C compiler
symbol __CEGCC__ is defined and the unix-like compatibility layer
is used. For our purposes this is not a native Windows build.

When cross compiling WinCE with the arm-wince-mingw32ce-gcc C compiler
symbol __MINGW32CE__ is defined and the unix-like compatibility layer
is not used. For our purposes this _is_ a native Windows build.