Respect CFLAGS/LDFLAGS and use DESTDIR for install targets.

--- libdvd-audio-1.0.0/Makefile
+++ libdvd-audio-1.0.0/Makefile
@@ -1,8 +1,9 @@
-FLAGS = -Wall -O2 -g -fPIC
-LIB_DIR = /usr/local/lib
-INCLUDE_DIR = /usr/local/include
-BIN_DIR = /usr/local/bin
-PKG_CONFIG_DIR = /usr/lib/pkgconfig
+FLAGS = -Wall -fPIC $(CFLAGS)
+PREFIX = /usr/local
+LIB_DIR = $(PREFIX)/lib
+INCLUDE_DIR = $(PREFIX)/include
+BIN_DIR = $(PREFIX)/bin
+PKG_CONFIG_DIR = $(LIB_DIR)/pkgconfig
 
 BITSTREAM_OBJS = bitstream.o \
 huffman.o \
@@ -56,13 +57,17 @@
 all: $(STATIC_LIBRARY) $(SHARED_LIBRARIES) $(BINARIES) $(PKG_CONFIG_METADATA)
 
 install: $(STATIC_LIBRARY) $(SHARED_LIBRARIES) $(BINARIES) $(PKG_CONFIG_METADATA)
-	install -m 644 $(SHARED_LIBRARY) $(LIB_DIR)
-	cp -Pp $(SHARED_LIBRARY_LINK_1) $(LIB_DIR)
-	cp -Pp $(SHARED_LIBRARY_LINK_2) $(LIB_DIR)
-	install -m 644 $(STATIC_LIBRARY) $(LIB_DIR)
-	install -m 644 include/dvd-audio.h $(INCLUDE_DIR)
-	install -m 755 $(BINARIES) $(BIN_DIR)
-	install -m 644 $(PKG_CONFIG_METADATA) $(PKG_CONFIG_DIR)
+	mkdir -p $(DESTDIR)$(LIB_DIR)
+	install -m 755 $(SHARED_LIBRARY) $(DESTDIR)$(LIB_DIR)
+	cp -Pp $(SHARED_LIBRARY_LINK_1) $(DESTDIR)$(LIB_DIR)
+	cp -Pp $(SHARED_LIBRARY_LINK_2) $(DESTDIR)$(LIB_DIR)
+	install -m 644 $(STATIC_LIBRARY) $(DESTDIR)$(LIB_DIR)
+	mkdir -p $(DESTDIR)$(INCLUDE_DIR)
+	install -m 644 include/dvd-audio.h $(DESTDIR)$(INCLUDE_DIR)
+	mkdir -p $(DESTDIR)$(BIN_DIR)
+	install -m 755 $(BINARIES) $(DESTDIR)$(BIN_DIR)
+	mkdir -p $(DESTDIR)$(PKG_CONFIG_DIR)
+	install -m 644 $(PKG_CONFIG_METADATA) $(DESTDIR)$(PKG_CONFIG_DIR)
 
 clean:
 	rm -f $(BINARIES) $(CODEBOOKS) $(BINARIES) $(PKG_CONFIG_METADATA) huffman *.o *.a *.so*
@@ -71,7 +76,7 @@
 	$(AR) -r $@ $(DVDA_OBJS)
 
 $(SHARED_LIBRARY): $(DVDA_OBJS)
-	$(CC) $(FLAGS) -Wl,-soname,libdvd-audio.so.$(MAJOR_VERSION) -shared -o $@ $(DVDA_OBJS)
+	$(CC) $(FLAGS) $(LDFLAGS) -Wl,-soname,libdvd-audio.so.$(MAJOR_VERSION) -shared -o $@ $(DVDA_OBJS)
 
 $(SHARED_LIBRARY_LINK_1): $(SHARED_LIBRARY)
 	ln -sf $< $@