Detecting 'make' environment variables change

While playing with 'iquik' and trying to add a mode to build a reduced-logging version that is smaller, I ran into an interesting question - how do I force a rebuild of everything with a clean?

#
# Example of a Makefile that detects "environment change".
#
# I.e.:
#
# andreiw-lnx:~/src/ make clean
# Cleaning
# andreiw-lnx:~/src/ make 
# Resuming build with env ""
# Building with ""
# andreiw-lnx:~/src/ make CONFIG_EXAMPLE=1
# Cleaning due to env change (was "" now "-DCONFIG_EXAMPLE")
# Cleaning
# Building with "-DCONFIG_EXAMPLE"
# andreiw-lnx:~/src/ make CONFIG_EXAMPLE=1
# Resuming build with env "-DCONFIG_EXAMPLE"
# Building with "-DCONFIG_EXAMPLE"
# andreiw-lnx:~/src
#

ENV_FILE=old_build_env
-include $(ENV_FILE)

#
# Environment definition.
#
ifeq ($(CONFIG_EXAMPLE), 1)
BUILD_FLAGS = -DCONFIG_EXAMPLE
endif
BUILD_ENV = "OLD_BUILD_FLAGS=$(BUILD_FLAGS)"

#
# Detect environment change.
#
ifneq ($(BUILD_FLAGS),$(OLD_BUILD_FLAGS))
 PRETARGET=clean_env
else
 PRETARGET=log_env
endif

all: $(PRETARGET) target

target:
 @echo Building with \"$(BUILD_FLAGS)\"

log_env:
 @echo Resuming build with env \"$(BUILD_FLAGS)\"

log_clean_env:
 @echo Cleaning due to env change \(was \"$(OLD_BUILD_FLAGS)\" now \"$(BUILD_FLAGS)\"\)

clean_env: log_clean_env clean
 @rm -f $(ENV_FILE)
 @echo $(BUILD_ENV) > $(ENV_FILE)

clean:
 @echo Cleaning