PKG_CONFIG ?= pkg-config
-CXXFLAGS := -std=gnu++14 -O2 -Wall -I. -g $(shell $(PKG_CONFIG) libusb-1.0 --cflags) -pthread
-LDFLAGS := $(shell $(PKG_CONFIG) libusb-1.0 --libs) -pthread
+CXXFLAGS ?= -O2 -g
+CXXFLAGS := -std=gnu++14 -Wall -I. $(shell $(PKG_CONFIG) libusb-1.0 --cflags) -pthread $(CXXFLAGS)
+LDFLAGS := $(shell $(PKG_CONFIG) libusb-1.0 --libs) -pthread $(LDFLAGS)
AR := ar
LN := ln
RANLIB := ranlib