gcc: fix 4.9.2 build with C++ and !wchar

The libcilk library (used on x86/x86-64 when building with C++ support)
unconditionally uses WCHAR_MIN / WCHAR_MAX, causing build issues with uClibc
when configured without wchar support.

Reported-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>