Remove bogus mingw & win16 ifdefs from plevent.h

Add stub declaration for _mbstr which isn't defined in the current mingw headers but is present in the libs.
Fix GRE definition of XPCOM_DLL for mingw.
Bug #134113 r=dougt

xpcom/build/nsXPCOMPrivate.h

@@ -113,12 +113,17 @@ NS_GetFrozenFunctions(XPCOMFunctions *entryPoints, const char* libraryPath);
  * XPCOM_DLL              - name of the loadable xpcom library on disk. 
  * XPCOM_SEARCH_KEY       - name of the environment variable that can be 
  *                          modified to include additional search paths.   
- * GRE_CONF_NAME          - Name of the GRE Configuration file                    
+ * GRE_CONF_NAME          - Name of the GRE Configuration file
  */
 
 #if defined(XP_WIN32) || defined(XP_OS2)
 
+#if defined(XP_WIN32) && defined(__GNUC__)
+#define XPCOM_DLL         "libxpcom" MOZ_DLL_SUFFIX
+#else
 #define XPCOM_DLL         "xpcom.dll"
+#endif
+
 #define XPCOM_SEARCH_KEY  "PATH"
 #define GRE_CONF_NAME     "gre.config"
 #define GRE_WIN_REG_LOC   "Software\\mozilla.org\\GRE\\"

xpcom/io/nsLocalFileWin.cpp

@@ -54,6 +54,13 @@
 
 #include "nsAutoLock.h"
 
+// _mbsstr isn't declared in w32api headers but it's there in the libs
+#ifdef __MINGW32__
+extern "C" {
+unsigned char *_mbsstr( const unsigned char *str,
+                        const unsigned char *substr );
+};
+#endif
 
 //----------------------------------------------------------------------------
 // short cut resolver

xpcom/threads/plevent.h

@@ -173,10 +173,8 @@ and to ensure that no more events will be delivered for that owner.
 #include "prmon.h"
 
 /* For HWND */
-#if defined(_WIN32) && !defined(__MINGW32__)
+#if defined(XP_WIN32)
 #include <windef.h>
-#elif defined(WIN16) || defined(__MINGW32__)
-#include <windows.h>
 #elif defined(XP_OS2)
 #define INCL_DOSMISC
 #define INCL_DOSPROCESS