move util.c to util.h

This allows single-source compilation and whole-program optimization using something like:

make CFLAGS='-flto -ffunction-sections -fdata-sections -Wl,--gc-sections'

Furthermore, move more compositor-agnostic utility functions/macros to util.h,
similar to the original dwm sources.

util.h

@@ -1,5 +1,58 @@
 /* See LICENSE.dwm file for copyright and license details. */
+#include <stdarg.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <fcntl.h>
 
-void die(const char *fmt, ...);
-void *ecalloc(size_t nmemb, size_t size);
-int fd_set_nonblock(int fd);
+#define MAX(A, B)               ((A) > (B) ? (A) : (B))
+#define MIN(A, B)               ((A) < (B) ? (A) : (B))
+#define LENGTH(X)               (sizeof X / sizeof X[0])
+#define END(A)                  ((A) + LENGTH(A))
+
+static void die(const char *fmt, ...);
+static void *ecalloc(size_t nmemb, size_t size);
+static int fd_set_nonblock(int fd);
+
+void
+die(const char *fmt, ...) {
+	va_list ap;
+
+	va_start(ap, fmt);
+	vfprintf(stderr, fmt, ap);
+	va_end(ap);
+
+	if (fmt[0] && fmt[strlen(fmt)-1] == ':') {
+		fputc(' ', stderr);
+		perror(NULL);
+	} else {
+		fputc('\n', stderr);
+	}
+
+	exit(1);
+}
+
+void *
+ecalloc(size_t nmemb, size_t size)
+{
+	void *p;
+
+	if (!(p = calloc(nmemb, size)))
+		die("calloc:");
+	return p;
+}
+
+int
+fd_set_nonblock(int fd) {
+	int flags = fcntl(fd, F_GETFL);
+	if (flags < 0) {
+		perror("fcntl(F_GETFL):");
+		return -1;
+	}
+	if (fcntl(fd, F_SETFL, flags | O_NONBLOCK) < 0) {
+		perror("fcntl(F_SETFL):");
+		return -1;
+	}
+
+	return 0;
+}