diff --git a/mk/platform.mk b/mk/platform.mk
index c032c04c1e2..e7b6cb57ef0 100644
--- a/mk/platform.mk
+++ b/mk/platform.mk
@@ -204,9 +204,15 @@ endif
 
 CFG_INFO := $(info cfg: using $(CFG_C_COMPILER))
 ifeq ($(CFG_C_COMPILER),clang)
-  CC ?= clang
-  CXX ?= clang++
-  CPP ?= cpp
+  ifeq ($(origin CC),default)
+    CC=clang
+  endif
+  ifeq ($(origin CXX),default)
+    CXX=clang++
+  endif
+  ifeq ($(origin CPP),default)
+    CPP=cpp
+  endif
   CFG_GCCISH_CFLAGS += -Wall -Werror -fno-rtti -g
   CFG_GCCISH_LINK_FLAGS += -g
   CFG_DEPEND_C = $(CFG_GCCISH_CROSS)$(CXX) $(CFG_GCCISH_CFLAGS) -MT "$(1)" \
@@ -229,9 +235,15 @@ ifeq ($(CFG_C_COMPILER),clang)
     $(eval $(call CFG_MAKE_CC,$(target))))
 else
 ifeq ($(CFG_C_COMPILER),gcc)
-  CC ?= gcc
-  CXX ?= g++
-  CPP ?= cpp
+  ifeq ($(origin CC),default)
+    CC=gcc
+  endif
+  ifeq ($(origin CXX),default)
+    CXX=g++
+  endif
+  ifeq ($(origin CPP),default)
+    CPP=cpp
+  endif
   CFG_GCCISH_CFLAGS += -Wall -Werror -fno-rtti -g
   CFG_GCCISH_LINK_FLAGS += -g
   CFG_DEPEND_C = $(CFG_GCCISH_CROSS)$(CXX) $(CFG_GCCISH_CFLAGS) -MT "$(1)" \