Makefile: keep track of environment variables

Previously, only values variables passed as Makefile variables
(make VAR=value) triggered rebuilds when changed. Now environment variables
(VAR=value make) are also tracked.

rf.: #1150

Author: L-series

test/mk/config.mk

@@ -78,9 +78,17 @@ endif
 # Include retained variables #
 ##############################
 
+RETAINED_VARS := CROSS_PREFIX CYCLES OPT AUTO
+
+# Capture values of environment variables before setting defaults,
+# this ensures we can detect when they change in order to trigger a rebuild.
+define CAPTURE_VAR
+$(1)_FROM_ENV := $$($(1))
+endef
+$(foreach var,$(RETAINED_VARS),$(eval $(call CAPTURE_VAR,$(var))))
+
 CYCLES ?=
 OPT ?= 1
-RETAINED_VARS := CROSS_PREFIX CYCLES OPT AUTO
 
 BUILD_DIR ?= test/build
 
@@ -91,6 +99,14 @@ CONFIG := $(BUILD_DIR)/config.mk
 
 -include $(CONFIG)
 
+# After including the cached config, restore environment/command-line values if they were set
+define RESTORE_VAR
+ifneq ($$($(1)_FROM_ENV),)
+  $(1) := $$($(1)_FROM_ENV)
+endif
+endef
+$(foreach var,$(RETAINED_VARS),$(eval $(call RESTORE_VAR,$(var))))
+
 $(CONFIG):
 	@echo "  GEN     $@"
 	$(Q)[ -d $(@D) ] || mkdir -p $(@D)