Respect CFLAGS/LDFLAGS/DESTDIR and don't try to run missing tests.

--- gif2png-2.5.11/Makefile
+++ gif2png-2.5.11/Makefile
@@ -2,8 +2,7 @@
 
 VERSION = 2.5.11
 
-CFLAGS = -DVERSION=\"$(VERSION)\" -O2
-LDFLAGS = -s
+CFLAGS += -DVERSION=\"$(VERSION)\"
 
 SOURCES = 437_l1.c gif2png.c gifread.c memory.c version.c
 OBJECTS = $(SOURCES:.c=.o)
@@ -28,19 +27,16 @@
 web2png.1: web2png.xml
 	xmlto man web2png.xml
 
-check:
-	cd test; make --quiet test
-
 clean:
 	rm -f gif2png *.o gif2png.1 web2png.1
 
-prefix?=/usr
+prefix ?= /usr
 
 install: gif2png gif2png.1 web2png.1
-	mkdir -p $(prefix)/bin
-	mkdir -p $(prefix)/share/man/man1
-	cp gif2png web2png $(prefix)/bin/
-	cp gif2png.1 web2png.1 $(prefix)/share/man/man1/
+	mkdir -p $(DESTDIR)$(prefix)/bin
+	mkdir -p $(DESTDIR)$(prefix)/share/man/man1
+	cp gif2png web2png $(DESTDIR)$(prefix)/bin/
+	cp gif2png.1 web2png.1 $(DESTDIR)$(prefix)/share/man/man1/
 
 uninstall:
 	rm $(prefix)/bin/gif2png $(prefix)/bin/web2png