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.

src/ca65/incpath.c

@@ -44,8 +44,8 @@
 
 
 
-SearchPath*     IncSearchPath;          /* Standard include path */
-SearchPath*     BinSearchPath;          /* Binary include path */
+SearchPaths*    IncSearchPath;          /* Standard include path */
+SearchPaths*    BinSearchPath;          /* Binary include path */
 
 
 
@@ -75,7 +75,7 @@ void FinishIncludePaths (void)
     AddSubSearchPathFromEnv (IncSearchPath, "CC65_HOME", "asminc");
 
     /* Add some compiled-in search paths if defined at compile time. */
-#ifdef CA65_INC
+#if defined(CA65_INC) && !defined(_WIN32)
     AddSearchPath (IncSearchPath, STRINGIZE (CA65_INC));
 #endif