Added a case to build_config.h to cover compiling for Linux with 16-bit wchars

(alongside the existing cases for Windows with 16-bit wchars and Linux with 
32-bit wchars) so that it becomes possible to compile and link the Chromium 
base together with third-party libraries that require short wchars. 

BUG=none
TEST=none
Original review http://codereview.chromium.org/315004
Patch by mdsteele@google.com

git-svn-id: http://src.chromium.org/svn/trunk/src/build@29786 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

build_config.h

@@ -96,6 +96,14 @@
     defined(__WCHAR_MAX__) && \
     (__WCHAR_MAX__ == 0x7fffffff || __WCHAR_MAX__ == 0xffffffff)
 #define WCHAR_T_IS_UTF32
+#elif defined(OS_POSIX) && defined(COMPILER_GCC) && \
+    defined(__WCHAR_MAX__) && \
+    (__WCHAR_MAX__ == 0x7fff || __WCHAR_MAX__ == 0xffff)
+// On Posix, we'll detect short wchar_t, but projects aren't guaranteed to
+// compile in this mode (in particular, Chrome doesn't). This is intended for
+// other projects using base who manage their own dependencies and make sure
+// short wchar works for them.
+#define WCHAR_T_IS_UTF16
 #else
 #error Please add support for your compiler in build/build_config.h
 #endif