-O6 Do not exist and is the same as -O3

Makefile.linux

@@ -6,7 +6,7 @@ ARCH=$(shell uname -m)
 DEFS=-DZT_ARCH="$(ARCH)" -DZT_OSNAME="linux" -DZT_TRACE
 
 # Uncomment for a release optimized build
-CFLAGS=-Wall -O6 -fno-unroll-loops -pthread $(INCLUDES) -DNDEBUG $(DEFS)
+CFLAGS=-Wall -O3 -fno-unroll-loops -pthread $(INCLUDES) -DNDEBUG $(DEFS)
 STRIP=strip --strip-all
 
 # Uncomment for a debug build