sys: newlib: check if toolchain is compiled with -fshort-wchar, set CFLAGS accordingly

sys/newlib/Makefile.include

@@ -4,6 +4,9 @@ ifneq (,$(filter newlib_nano,$(USEMODULE)))
   # Test if nano.specs is available
   ifeq ($(shell $(LINK) -specs=nano.specs -E - 2>/dev/null >/dev/null </dev/null ; echo $$?),0)
     USE_NEWLIB_NANO = 1
+    ifeq ($(shell echo "int main(){} void _exit(int n) {(void)n;while(1);}" | LC_ALL=C $(CC) -xc - -o /dev/null -lc -specs=nano.specs -Wall -Wextra -pedantic 2>&1 | grep -q "use of wchar_t values across objects may fail" ; echo $$?),0)
+        CFLAGS += -fshort-wchar
+    endif
   endif
 endif