Improve MinGW support.

- Code specific to Windows was #ifdef'ed with _MSC_VER so it wasn't included with MinGW. So _MSC_VER is replaced with _WIN32.
- MinGW doesn't support _get_pgmptr() so it is necessary to directly call the Win32 function GetModuleFileName(). This implies including windows.h which in turn causes a name clash with the Win32 function SearchPath(). So the cc65 type SearchPath is renamed to SearchPaths.
Adjusted spawn files to recent change.
Adjusted spawn files to recent change II.
Removed unused header.
Normalized code.
Moved workaround for MinGW's missing %m support to a central place.
Merge remote-tracking branch 'upstream/master'