Ignore set but empty CORES variable

Change the behavior when the CORES environment variable
is set and empty.
Now `-j` will not be added to make/ninja.

build-llvm.sh

@@ -309,6 +309,6 @@ cmake \
     $CMAKEFLAGS \
     ..
 
-$BUILDCMD ${CORES+-j$CORES} install/strip
+$BUILDCMD ${CORES:+-j${CORES}} install/strip
 
 cp ../LICENSE.TXT $PREFIX