Add features to Makefile

make release - compiles with clang, uses more aggressive compilation
flags for errors/warnings.
CC=tcc/gcc/clang make - lets you select your wanted compiler.

src/util.h

@@ -1,5 +1,5 @@
 /* See LICENSE.dwm file for copyright and license details. */
 
-void die(const char *fmt, ...);
+_Noreturn void die(const char *fmt, ...);
 void *ecalloc(size_t nmemb, size_t size);
 int fd_set_nonblock(int fd);