Add logging functionality

This is logging functionality for weston compositor.

It handles:
messages coming from libwayland-server from wl_log()
messages from weston itself, from weston_log()

Introduce --log option, to specify log file path on the command line.
When the path is incorrect, or on weston_log_file_destroy(), fall
back to stderr.
diff --git a/src/log.c b/src/log.c
new file mode 100644
index 0000000..0042619
--- /dev/null
+++ b/src/log.c
@@ -0,0 +1,115 @@
+/*
+ * Copyright © 2012 Martin Minarik
+ *
+ * Permission to use, copy, modify, distribute, and sell this software and
+ * its documentation for any purpose is hereby granted without fee, provided
+ * that the above copyright notice appear in all copies and that both that
+ * copyright notice and this permission notice appear in supporting
+ * documentation, and that the name of the copyright holders not be used in
+ * advertising or publicity pertaining to distribution of the software
+ * without specific, written prior permission.  The copyright holders make
+ * no representations about the suitability of this software for any
+ * purpose.  It is provided "as is" without express or implied warranty.
+ *
+ * THE COPYRIGHT HOLDERS DISCLAIM ALL WARRANTIES WITH REGARD TO THIS
+ * SOFTWARE, INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND
+ * FITNESS, IN NO EVENT SHALL THE COPYRIGHT HOLDERS BE LIABLE FOR ANY
+ * SPECIAL, INDIRECT OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER
+ * RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF
+ * CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
+ * CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+
+#include <stdio.h>
+#include <stdarg.h>
+#include <stdlib.h>
+#include <string.h>
+#include <time.h>
+
+#include <wayland-server.h>
+#include <wayland-util.h>
+
+#include "log.h"
+
+static FILE *weston_logfile = NULL;
+
+static char cached_timestamp[21];
+static int cached_tv_sec = 0;
+
+static int weston_log_timestamp(void)
+{
+	struct timespec tp;
+	unsigned int time;
+
+	clock_gettime(CLOCK_REALTIME, &tp);
+	time = (tp.tv_sec * 1000000L) + (tp.tv_nsec / 1000);
+
+	if (cached_tv_sec != tp.tv_sec) {
+		char string[26];
+		struct tm *lt = localtime(&tp.tv_sec);
+
+		cached_tv_sec = tp.tv_sec;
+
+		strftime (string,24,"%Y-%m-%d %H:%M:%S", lt);
+		strncpy (cached_timestamp, string, 20);
+	}
+
+	return fprintf(weston_logfile, "[%s.%03u] ", cached_timestamp, tp.tv_nsec/1000000);
+}
+
+static int weston_log_print(const char *fmt, va_list arg)
+{
+	int l;
+	l = vfprintf(weston_logfile, fmt, arg);
+	fflush(weston_logfile);
+	return l;
+}
+
+static void
+custom_handler(const char *fmt, va_list arg)
+{
+	weston_log_timestamp();
+	fprintf(weston_logfile, "libwayland: ");
+	weston_log_print(fmt, arg);
+}
+
+void
+weston_log_file_open(const char *filename)
+{
+	wl_log_set_handler_server(custom_handler);
+
+	weston_logfile = fopen(filename, "a");
+	if (weston_logfile == NULL)
+		weston_logfile = stderr;
+}
+
+void
+weston_log_file_close()
+{
+	if (weston_logfile != stderr)
+		fclose(weston_logfile);
+	weston_logfile = stderr;
+}
+
+WL_EXPORT int
+weston_log(const char *fmt, ...)
+{
+	int l;
+	va_list argp;
+	va_start(argp, fmt);
+	l = weston_log_timestamp();
+	l += weston_log_print(fmt, argp);
+	va_end(argp);
+	return l;
+}
+
+WL_EXPORT int
+weston_log_continue(const char *fmt, ...)
+{
+	int l;
+	va_list argp;
+	va_start(argp, fmt);
+	l = weston_log_print(fmt, argp);
+	va_end(argp);
+	return l;
+}