Add Watcom C/C++ calling.

In the unlikely case the source is built with OpenWatcom's -ec?
switches to enforce a calling convention, the qsort() compare
function must still be set to __watcall.

* include/freetype/internal/compiler-macros.h (FT_COMPARE_DEF):
Updated.

include/freetype/internal/compiler-macros.h

@@ -303,6 +303,8 @@ FT_BEGIN_HEADER
 #define FT_COMPARE_DEF( x )  FT_CALLBACK_DEF( x ) __attribute__(( cdecl ))
 #elif defined( _MSC_VER ) && defined( _M_IX86 )
 #define FT_COMPARE_DEF( x )  FT_CALLBACK_DEF( x ) __cdecl
+#elif defined( __WATCOMC__ ) && __WATCOMC__ >= 1240
+#define FT_COMPARE_DEF( x )  FT_CALLBACK_DEF( x ) __watcall
 #else
 #define FT_COMPARE_DEF( x )  FT_CALLBACK_DEF( x )
 #endif