Append to MacPorts CFLAGS and CXXFLAGS rather than overwriting them.
--- Makefile.orig 2019-01-08 17:13:16.000000000 -0600
+++ Makefile 2019-01-08 17:32:37.000000000 -0600
# -s silcently corrupts binaries on OS X, sigh -ReneR
-CFLAGS := -Wall -O2 # -O1 -ggdb # -fsanitize=address -fsanitize=undefined
cc-option = $(shell if $(CC) $(1) -S -o /dev/null -xc /dev/null \
> /dev/null 2>&1; then echo "$(1)"; else echo "$(2)"; fi ;)
-CFLAGS += -funroll-loops -fomit-frame-pointer
+COMPILER_FLAGS += -funroll-loops -fomit-frame-pointer
# we have some unimplemented colorspaces in the Image::iterator :-(
-CFLAGS += $(call cc-option,-Wno-switch -Wno-switch-enum,)
+COMPILER_FLAGS += $(call cc-option,-Wno-switch -Wno-switch-enum,)
-CXXFLAGS := $(CFLAGS) -Wno-sign-compare
+CFLAGS += $(COMPILER_FLAGS)
+CXXFLAGS += $(COMPILER_FLAGS) -Wno-sign-compare