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.

Signed-off-by: Andreas Hatziiliou <andreas.hatziiliou@savoirfairelinux.com>

Author: Andreas

test/mk/config.mk

@@ -80,10 +80,19 @@ 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))))
+
 AUTO ?= 1
 CYCLES ?=
 OPT ?= 1
-RETAINED_VARS := CROSS_PREFIX CYCLES OPT AUTO
 
 ifeq ($(AUTO),1)
 include test/mk/auto.mk
@@ -98,6 +107,14 @@ 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)