diff -Naurp a/src/Makefile b/src/Makefile
--- a/src/Makefile	2017-04-28 15:46:54.000000000 +0000
+++ b/src/Makefile	2018-09-13 18:37:05.277277156 +0000
@@ -1,6 +1,6 @@
 PREFIX := $(DESTDIR)/usr/local
-CXXFLAGS := $(shell pkg-config --cflags zlib libpng) -DLODEPNG_NO_COMPILE_PNG -DLODEPNG_NO_COMPILE_DISK
-LDFLAGS := $(shell pkg-config --libs libpng)
+CXXFLAGS := $(CXXFLAGS) $(shell pkg-config --cflags zlib libpng) -DLODEPNG_NO_COMPILE_PNG -DLODEPNG_NO_COMPILE_DISK
+LDFLAGS := $(LDFLAGS) $(shell pkg-config --libs libpng)
 
 OSNAME := $(shell uname -s)
 SONAME = -soname
@@ -14,7 +14,7 @@ ifeq ($(OSNAME),Darwin)
 endif
 
 # for running interface-test
-export LD_LIBRARY_PATH=$(shell pwd):/usr/local/lib:$LD_LIBRARY_PATH
+export LD_LIBRARY_PATH := $(shell pwd):/usr/local/lib:$(LD_LIBRARY_PATH)
 
 FILES_H := maniac/*.hpp maniac/*.cpp image/*.hpp transform/*.hpp flif-enc.hpp flif-dec.hpp common.hpp flif_config.h fileio.hpp io.hpp io.cpp config.h compiler-specific.hpp ../extern/lodepng.h
 FILES_CPP := maniac/chance.cpp maniac/symbol.cpp image/crc32k.cpp image/image.cpp image/image-png.cpp image/image-pnm.cpp image/image-pam.cpp image/image-rggb.cpp image/image-metadata.cpp image/color_range.cpp transform/factory.cpp common.cpp flif-enc.cpp flif-dec.cpp io.cpp ../extern/lodepng.cpp
@@ -36,7 +36,7 @@ endif
 
 # Command-line FLIF encoding/decoding tool - LGPLv3
 flif: $(FILES_H) $(FILES_CPP) flif.cpp
-	$(CXX) -std=gnu++11 $(CXXFLAGS) $(OPTIMIZATIONS) -g0 -Wall $(FILES_CPP) flif.cpp $(LDFLAGS) -o flif
+	$(CXX) -std=gnu++11 $(CXXFLAGS) $(CPPFLAGS) $(OPTIMIZATIONS) -g0 -Wall $(FILES_CPP) flif.cpp $(LDFLAGS) -o flif
 
 # Command-line FLIF decoding tool - Apache2 (not built by default)
 dflif: $(FILES_H) $(FILES_CPP) flif.cpp
@@ -50,7 +50,7 @@ libflif_dec$(LIBEXT): $(FILES_H) $(FILES
 
 # Decoder + encoder library - LGPL
 libflif$(LIBEXT): $(FILES_H) $(FILES_CPP) library/*.h library/*.hpp library/*.cpp
-	$(CXX) -std=gnu++11 $(CXXFLAGS) $(LIB_OPTIMIZATIONS) -g0 -Wall -shared -fPIC $(FILES_CPP) library/flif-interface.cpp $(LDFLAGS) -Wl,$(SONAME),libflif$(LIBEXTV) -o libflif$(LIBEXTV)
+	$(CXX) -std=gnu++11  $(CPPFLAGS) $(CXXFLAGS) $(LIB_OPTIMIZATIONS) -g0 -Wall -shared -fPIC $(FILES_CPP) library/flif-interface.cpp $(LDFLAGS) -Wl,$(SONAME),libflif$(LIBEXTV) -o libflif$(LIBEXTV)
 	ln -sf libflif$(LIBEXTV) libflif$(LIBEXT)
 
 libflif.dbg$(LIBEXT): $(FILES_H) $(FILES_CPP) library/*.h library/*.hpp library/*.cpp
@@ -59,7 +59,7 @@ libflif.dbg$(LIBEXT): $(FILES_H) $(FILES
 
 # Example application: simple FLIF viewer - public domain
 viewflif: libflif$(LIBEXT) viewflif.c
-	$(CC) -std=gnu11 -O3 -ggdb3 $(shell sdl2-config --cflags) -Wall -Ilibrary/ viewflif.c -L. -lflif $(shell sdl2-config --libs) -o viewflif
+	$(CC) -std=gnu11 $(CPPFLAGS) $(CXXFLAGS) $(LDFLAGS) -O3 -ggdb3 $(shell sdl2-config --cflags) -Wall -Ilibrary/ viewflif.c -L. -lflif $(shell sdl2-config --libs) -o viewflif
 
 
 install: flif libflif$(LIBEXT)