Avoid wchar_t redefinition

The musl C library does not define _WCHAR_T. Check also for compiler defined
__WCHAR_TYPE__.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>

diff -Nuar ipmiutil-2.9.5-orig/util/imb_api.h ipmiutil-2.9.5/util/imb_api.h
--- ipmiutil-2.9.5-orig/util/imb_api.h	2014-11-04 19:46:11.000000000 +0200
+++ ipmiutil-2.9.5/util/imb_api.h	2015-09-29 07:49:40.799063825 +0300
@@ -49,7 +49,7 @@
 /* DOS defines wchar_t in stdlib.h */
 #else 
  // defined(LINUX) | defined(SOLARIS)
-#ifndef _WCHAR_T
+#if !defined(_WCHAR_T) && !defined(__WCHAR_TYPE__)
 #define _WCHAR_T
 typedef long    wchar_t;
 #endif