package/gcc: complete the logic on libquadmath

libquadmath requires wchar.

So, turn to positive logic and complete it to only enabling quadmath
support when it is available.

Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/gcc/gcc.mk

@@ -115,7 +115,9 @@ HOST_GCC_COMMON_CONF_OPTS += --disable-libitm
 endif
 
 # quadmath support requires wchar
-ifeq ($(BR2_USE_WCHAR),)
+ifeq ($(BR2_USE_WCHAR)$(BR2_TOOLCHAIN_HAS_LIBQUADMATH),yy)
+HOST_GCC_COMMON_CONF_OPTS += --enable-libquadmath
+else
 HOST_GCC_COMMON_CONF_OPTS += --disable-libquadmath
 endif