library optimization flag can be selected on command line again

`CFLAGS=-O0 make`
will now use `-O0` instead of enforcing `-O3`
which used to be the behavior before introduction of `libzstd.mk`.

This should result in faster tests,
since a few tests depend on this capability for faster roundtrips.

lib/libzstd.mk

@@ -79,7 +79,7 @@ endif
   CFLAGS += -fno-stack-protector -fomit-frame-pointer -fno-ident \
             -DDYNAMIC_BMI2=0 -DNDEBUG
 else
-  CFLAGS += -O3
+  CFLAGS ?= -O3
 endif
 
 DEBUGLEVEL ?= 0