fix: GNU Make predefines variables

This causes issues leading to the various flags and variables being set
to unexpected values.

Makefile

@@ -2,13 +2,12 @@ SHELL = /bin/sh
 .SUFFIXES:
 
 CC     = avr-gcc
-RM     = rm -vf
 STRIP  = avr-strip
 
-CPPFLAGS ?= -DNDEBUG
-CFLAGS   ?= -Os
-LDFLAGS  ?= -nostdlib -nostartfiles -flto
+CPPFLAGS = -DNDEBUG
+CFLAGS   = -Os
 
+LDFLAGS  += -nostdlib -nostartfiles -flto
 CPPFLAGS += -Iinclude
 CFLAGS   += -std=gnu99 -MMD -MP
 CFLAGS   += -Wall -Wextra -Wpedantic -Wno-pointer-arith
@@ -18,20 +17,27 @@ SRC += $(wildcard src/*.S src/*/*.S src/*/*/*.S src/*/*/*/*.S src/*/*/*/*/*.S sr
 OBJ := $(SRC:%=obj/%.o)
 
 
-all:    bin/a.out
-clean:; @-$(RM) -r bin/ obj/
+.PHONY: all clean install-strip info
+all: info bin/a.out
+clean:; @-$(RM) -rv bin/ obj/
+info:
+	$(info CC:       $(CC))
+	$(info CFLAGS:   $(CFLAGS))
+	$(info CPPFLAGS: $(CPPFLAGS))
+	$(info LDFLAGS:  $(LDFLAGS))
+	$(info LDLIBS:   $(LDLIBS))
 
 bin/a.out: $(OBJ)
-	$(info [LD]	$@)
+	$(info [LD]	$(CC)	$@)
 	@mkdir -p $(@D)
 	@$(CC) -o $@ $^ $(LDFLAGS) $(LDLIBS)
 
 bin/a.hex: bin/a.out
-	$(info [STRIP]	$@)
+	$(info [STRIP]	$(STRIP)	$@)
 	@$(STRIP) -o $@ $<
 
 obj/%.o: %
-	$(info [CC]	$@)
+	$(info [CC]	$(CC)	$@)
 	@mkdir -p $(@D)
 	@$(CC) -c $(CPPFLAGS) $(CFLAGS) -o $@ $<