Support building with other compilers (e.g, Clang)

diff -urN a/Makefile b/Makefile
--- a/Makefile	2003-05-01 21:50:38.000000000 +0000
+++ b/Makefile	2025-11-27 07:12:59.754162192 +0000
@@ -1,5 +1,5 @@
 all: nrg2iso.c
-	gcc nrg2iso.c -o nrg2iso
+	$(CC) nrg2iso.c -o nrg2iso
 
 clean: 
 	rm -f nrg2iso