Get Makefile working with both C and C++ compilers

We need HAVE_STPCPY defined to avoid duplicating it.

We shouldn't depend on CPPFLAGS if building with gcc.
It may be necessary to build with a C++ compiler when using portaudio
or integrating into a C++ project. Now it builds either way.

Makefile

@@ -11,13 +11,15 @@ FFT_OBJ  = $(patsubst %.c,$(BUILD_DIR)/%.o,$(FFT_SRC))
 
 TARGETS  = gen_ft8 decode_ft8 test_ft8
 
-CFLAGS   = -fsanitize=address -O3 -ggdb3
-CPPFLAGS = -std=c11 -I.
+CFLAGS   = -fsanitize=address -O3 -ggdb3 -DHAVE_STPCPY -I.
 LDFLAGS  = -fsanitize=address -lm
 
 # Optionally, use Portaudio for live audio input
+# Portaudio is a C++ library, so then you need to build with clang++ or g++:
+# it's up to you to set CC accordingly!
+# Then -std=c11 is useful.
 ifdef PORTAUDIO_PREFIX
-CPPFLAGS += -DUSE_PORTAUDIO -I$(PORTAUDIO_PREFIX)/include
+CFLAGS += -std=c11 -DUSE_PORTAUDIO -I$(PORTAUDIO_PREFIX)/include
 LDFLAGS  += -lportaudio -L$(PORTAUDIO_PREFIX)/lib
 endif
 
@@ -46,4 +48,4 @@ test_ft8: $(BUILD_DIR)/test/test.o $(FT8_OBJ)
 
 $(BUILD_DIR)/%.o: %.c
 	@mkdir -p $(dir $@)
-	$(CC) $(CFLAGS) $(CPPFLAGS) -o $@ -c $^
+	$(CC) $(CFLAGS) -o $@ -c $^