Support building with compilers such as Clang.

--- a/Makefile
+++ b/Makefile
@@ -1,5 +1,5 @@
 all: nrg2iso.c
-	gcc nrg2iso.c -o nrg2iso
+	cc nrg2iso.c -o nrg2iso

 clean:
 	rm -f nrg2iso