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.

diff --git a/lib/libzstd.mk b/lib/libzstd.mk
index af12daffe12814ef9299d41b5a9884af0e03da7a..c04957c6faef20fcb003f7c2fd4a0457f137be79 100644 (file)
--- a/lib/libzstd.mk
+++ b/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