make: use ccache if environment variable is set

Makefile.base

@@ -57,13 +57,13 @@ CXXFLAGS = $(filter-out $(CXXUWFLAGS), $(CFLAGS)) $(CXXEXFLAGS)
 # compile and generate dependency info
 
 $(OBJC): $(BINDIR)$(MODULE)/%.o: %.c
-	$(AD)$(CC) \
+	$(AD)$(CCACHE) $(CC) \
 		-DRIOT_FILE_RELATIVE=\"$(patsubst $(RIOTBASE)/%,%,$(abspath $<))\" \
 		-DRIOT_FILE_NOPATH=\"$(notdir $<)\" \
 		$(CFLAGS) $(INCLUDES) -MD -MP -c -o $@ $(abspath $<)
 
 $(OBJCXX): $(BINDIR)$(MODULE)/%.o: %.cpp
-	$(AD)$(CXX) \
+	$(AD)$(CCACHE) $(CXX) \
 		-DRIOT_FILE_RELATIVE=\"$(patsubst $(RIOTBASE)/%,%,$(abspath $<))\" \
 		-DRIOT_FILE_NOPATH=\"$(notdir $<)\" \
 		$(CXXFLAGS) $(INCLUDES) -MD -MP -c -o $@ $(abspath $<)