Respect CXX, CXXFLAGS, CPPFLAGS, and PKG_CONFIG.
Add way to choose audio backend (alsa, pulse, or neither).

Alternatively, cmake migration has been proposed upstream:
https://gitlab.com/rastersoft/fbzx/-/merge_requests/10
--- a/src/Makefile
+++ b/src/Makefile
@@ -3,8 +3,14 @@
 CC=g++ -c $(FLAGS)
-CXX=g++ -c $(FLAGS)
-LN=g++
+LN=$(CXX)
 
-CFLAGS += `pkg-config --cflags sdl2 libpulse-simple alsa` -D D_SOUND_PULSE -D D_SOUND_ALSA
-CXXFLAGS += `pkg-config --cflags sdl2 libpulse-simple alsa` -D D_SOUND_PULSE -D D_SOUND_ALSA
-LDFLAGS += `pkg-config --libs sdl2 libpulse-simple alsa`
+CXXFLAGS:=-c $(CXXFLAGS) $(CPPFLAGS) $(shell $(PKG_CONFIG) --cflags sdl2)
+LDFLAGS:=$(LDFLAGS) $(shell $(PKG_CONFIG) --libs sdl2)
+ifeq ($(ALSA),yes)
+	CXXFLAGS+=$(shell $(PKG_CONFIG) --cflags alsa) -DD_SOUND_ALSA
+	LDFLAGS+=$(shell $(PKG_CONFIG) --libs alsa)
+endif
+ifeq ($(PULSE),yes)
+	CXXFLAGS+=$(shell $(PKG_CONFIG) --cflags libpulse-simple) -DD_SOUND_PULSE
+	LDFLAGS+=$(shell $(PKG_CONFIG) --libs libpulse-simple)
+endif
 
@@ -15,24 +21,24 @@
 Z80free.o: z80free/Z80free.c z80free/Z80free.h
-	$(CXX) -o Z80free.o z80free/Z80free.c
+	$(CXX) $(CXXFLAGS) -o Z80free.o z80free/Z80free.c
 
 Z80free_codes.o: z80free/Z80free_codes.c z80free/Z80free.h
-	$(CXX) -o Z80free_codes.o z80free/Z80free_codes.c
+	$(CXX) $(CXXFLAGS) -o Z80free_codes.o z80free/Z80free_codes.c
 
 Z80free_codesCB.o: z80free/Z80free_codesCB.c z80free/Z80free.h
-	$(CXX) -o Z80free_codesCB.o z80free/Z80free_codesCB.c
+	$(CXX) $(CXXFLAGS) -o Z80free_codesCB.o z80free/Z80free_codesCB.c
 
 Z80free_codesED.o: z80free/Z80free_codesED.c z80free/Z80free.h
-	$(CXX) -o Z80free_codesED.o z80free/Z80free_codesED.c
+	$(CXX) $(CXXFLAGS) -o Z80free_codesED.o z80free/Z80free_codesED.c
 
 Z80free_codesDD.o: z80free/Z80free_codesDD.c z80free/Z80free.h
-	$(CXX) -o Z80free_codesDD.o z80free/Z80free_codesDD.c
+	$(CXX) $(CXXFLAGS) -o Z80free_codesDD.o z80free/Z80free_codesDD.c
 
 Z80free_codesFD.o: z80free/Z80free_codesFD.c z80free/Z80free.h
-	$(CXX) -o Z80free_codesFD.o z80free/Z80free_codesFD.c
+	$(CXX) $(CXXFLAGS) -o Z80free_codesFD.o z80free/Z80free_codesFD.c
 
 Z80free_codesDDCB.o: z80free/Z80free_codesDDCB.c z80free/Z80free.h
-	$(CXX) -o Z80free_codesDDCB.o z80free/Z80free_codesDDCB.c
+	$(CXX) $(CXXFLAGS) -o Z80free_codesDDCB.o z80free/Z80free_codesDDCB.c
 
 Z80free_codesFDCB.o: z80free/Z80free_codesFDCB.c z80free/Z80free.h
-	$(CXX) -o Z80free_codesFDCB.o z80free/Z80free_codesFDCB.c
+	$(CXX) $(CXXFLAGS) -o Z80free_codesFDCB.o z80free/Z80free_codesFDCB.c