diff --git a/config.mk b/config.mk
index 314d6bb..417c0c1 100644
--- a/config.mk
+++ b/config.mk
@@ -6,7 +6,7 @@ PREFIX ?= $(HOME)/.idris2
 OPT =
 
 # clang compiles the output much faster than gcc!
-CC ?= clang
+CC := clang
 
 ##################################################################
 
diff --git a/dist/config.mk b/dist/config.mk
index 3c502b5..f5eae67 100644
--- a/dist/config.mk
+++ b/dist/config.mk
@@ -1,6 +1,6 @@
 include $(IDRIS2_CURDIR)/config.mk
 
-CFLAGS += -O2 -std=c99 -pipe -fdata-sections -ffunction-sections -D_POSIX_C_SOURCE=200809L
+CFLAGS += $(OPT) -std=c99 -pipe -fdata-sections -ffunction-sections -D_POSIX_C_SOURCE=200809L
 
 ifeq ($(OS),bsd)
 	GMP_INCLUDE_DIR = -I/usr/local/include
diff --git a/dist/rts/Makefile b/dist/rts/Makefile
index 608d9d0..a3b55d4 100644
--- a/dist/rts/Makefile
+++ b/dist/rts/Makefile
@@ -7,7 +7,7 @@ HDRS =idris_rts.h idris_heap.h idris_gc.h idris_gmp.h idris_bitstring.h \
       idris_opts.h idris_stats.h idris_stdfgn.h idris_net.h \
       idris_buffer.h idris_utf8.h getline.h
 
-CFLAGS += $(GMP_INCLUDE_DIR) -DIDRIS_TARGET_OS="\"$(OS)\""
+CFLAGS += -O2 $(GMP_INCLUDE_DIR) -DIDRIS_TARGET_OS="\"$(OS)\""
 CFLAGS += -DIDRIS_TARGET_TRIPLE="\"$(MACHINE)\""
 
 ifeq ($(OS), windows)