Index: phobos/posix.mak
--- phobos/posix.mak.orig
+++ phobos/posix.mak
@@ -107,15 +107,13 @@ endif
 OUTFILEFLAG = -o
 NODEFAULTLIB=-defaultlib= -debuglib=
 ifeq (,$(findstring win,$(OS)))
-	CFLAGS=$(MODEL_FLAG) -fPIC -std=c11 -DHAVE_UNISTD_H
+	CFLAGS+=$(MODEL_FLAG) -fPIC -std=c11 -DHAVE_UNISTD_H
 # Bundled with the system library on OSX, and doesn't work with >= MacOS 11
 	ifneq (osx,$(OS))
 		NODEFAULTLIB += -L-lpthread -L-lm
 	endif
 	ifeq ($(BUILD),debug)
 		CFLAGS += -g
-	else
-		CFLAGS += -O3
 	endif
 else
 	ifeq ($(OS),win32)
