package/gcc: use BR2_USE_WCHAR to set gcc libquadmath option

BR2_TOOLCHAIN_BUILDROOT_WCHAR is only defined when uclibc is selected, whereas
BR2_USE_WCHAR is always defined.

So, use BR2_USE_WCHAR to drive the gcc libquadmath option.

Signed-off-by: Samuel Martin <s.martin49@gmail.com>

package/gcc/gcc.mk

@@ -114,7 +114,7 @@ HOST_GCC_COMMON_CONF_OPTS += --disable-libitm
 endif
 
 # gcc 4.6.x quadmath requires wchar
-ifneq ($(BR2_TOOLCHAIN_BUILDROOT_WCHAR),y)
+ifneq ($(BR2_USE_WCHAR),y)
 HOST_GCC_COMMON_CONF_OPTS += --disable-libquadmath
 endif