Improvement #74: Use libevent for main loop

- Use libevent for main loop. I will explain the reasons in #56 later.
  The preferred libevent version is 2.x, yet 1.4.x should work as well.

- As a result, compton now should build fine on *BSD. Thanks to
  DachiChang for the FreeBSD build issue report.

- Another consequence is we now use microsecond-level timing all the
  way. Nanosecond-level code will be dropped soon. Start using long
  instead of unsigned long to represent time in milliseconds, as both
  can't hold the full epoch time in ms, anyway, and a signed type
  requires less care in subtraction. Wrap the epoch time in ms to 15
  days.

- Fix broken NO_VSYNC_DRM and NO_VSYNC_OPENGL compile-time options.

- Use git revision number for versioning in Makefile, and other small
  improvements.

- Reorganize struct _win. Drop unused w->damaged_sequence. w->damaged is
  turned to bool.

- Add type and format to winprop_t, as preparation for the new condition
  format.

- Add w->shadow_force and w->focus_force, to prepare for D-Bus support.

- Rename wid_get_prop() to wid_get_prop_adv() with more options. Add
  wrapper function wid_get_prop().

- Add some extra helper functions, for D-Bus support later.

- Make some functions return a bool value to indicate if it's
  successful.

- Modify add_win(), use a static const structure to initialize the new
  struct _win.

- Add some helper macros, like printf_err(f)(q). Make some errors fatal.

- Rename some types, constants, and functions. Code clean-up.

- Check for time disorder in paint_preprocess() when calculating fading
  steps.

- Rename evpoll() to swopti_handle_timeout(), and partially rewrite it.

- Make -h / --help legal.

- Known issue: compton segfaults on FreeBSD with nvidia-drivers, unless
  NO_VSYNC_OPENGL is used. Will look into it later. Thamls to DachiChang
  for reporting.

Makefile

@@ -1,3 +1,6 @@
+# Use tab to indent recipe lines, spaces to indent other lines, otherwise
+# GNU make may get unhappy.
+
 CC ?= gcc
 
 PREFIX ?= /usr
@@ -8,44 +11,79 @@ PACKAGES = x11 xcomposite xfixes xdamage xrender xext xrandr
 LIBS = -lm -lrt
 INCS =
 
-# Parse configuration flags
+# === Configuration flags ===
 CFG =
 
+# ==== libevent ====
+# Seemingly, libevent1 has no pkg-config file!
+# FreeBSD 9.1 probably has issues handling --atleast-version in pkg-config.
+ifeq "$(shell pkg-config --modversion --print-errors libevent)" ""
+  $(warning libevent-2.0 not found, assuming libevent-1.4.x.)
+  CFG += -DCONFIG_LIBEVENT_LEGACY
+  LIBS += -levent
+else
+  # Using pkg-config for linking with libevent will result in linking with
+  # libevent.so instead of the smaller libevent_core.so. But FreeBSD keeps
+  # libevent2 .so files at a separate place, and we must learn it from
+  # pkg-config.
+  LIBS += $(shell pkg-config --libs libevent)
+  INCS += $(shell pkg-config --cflags libevent)
+endif
+
+# ==== libconfig ====
 ifeq "$(NO_LIBCONFIG)" ""
-	CFG += -DCONFIG_LIBCONFIG
-	PACKAGES += libconfig
+  CFG += -DCONFIG_LIBCONFIG
+  PACKAGES += libconfig
 
-	# libconfig-1.3* does not define LIBCONFIG_VER* macros, so we use
-	# pkg-config to determine its version here
-	CFG += $(shell pkg-config --atleast-version=1.4 libconfig || echo '-DCONFIG_LIBCONFIG_LEGACY')
+  # libconfig-1.3* does not define LIBCONFIG_VER* macros, so we use
+  # pkg-config to determine its version here
+  CFG += $(shell pkg-config --atleast-version=1.4 libconfig || echo '-DCONFIG_LIBCONFIG_LEGACY')
 endif
 
+# ==== PCRE regular expression ====
 ifeq "$(NO_REGEX_PCRE)" ""
-	CFG += -DCONFIG_REGEX_PCRE
-	LIBS += $(shell pcre-config --libs)
-	INCS += $(shell pcre-config --cflags)
-	ifeq "$(NO_REGEX_PCRE_JIT)" ""
-		CFG += -DCONFIG_REGEX_PCRE_JIT
-	endif
+  CFG += -DCONFIG_REGEX_PCRE
+  LIBS += $(shell pcre-config --libs)
+  INCS += $(shell pcre-config --cflags)
+  ifeq "$(NO_REGEX_PCRE_JIT)" ""
+    CFG += -DCONFIG_REGEX_PCRE_JIT
+  endif
 endif
 
+# ==== DRM VSync ====
 ifeq "$(NO_VSYNC_DRM)" ""
-	CFG += -DCONFIG_VSYNC_DRM
+  CFG += -DCONFIG_VSYNC_DRM
 endif
 
+# ==== OpenGL VSync ====
 ifeq "$(NO_VSYNC_OPENGL)" ""
-	CFG += -DCONFIG_VSYNC_OPENGL
-	LIBS += -lGL
+  CFG += -DCONFIG_VSYNC_OPENGL
+  LIBS += -lGL
 endif
 
-CFLAGS ?= -DNDEBUG -O2 -D_FORTIFY_SOURCE=2
+# ==== D-Bus ====
+# ifeq "$(NO_DBUS)" ""
+#   CFG += -DCONFIG_DBUS
+#   PACKAGES += dbus-1
+# endif
+
+# === Version string ===
+COMPTON_VERSION ?= git-$(shell git describe --always)
+CFG += -DCOMPTON_VERSION="\"$(COMPTON_VERSION)\""
+
+LDFLAGS ?= -Wl,-O1 -Wl,--as-needed
+CFLAGS ?= -DNDEBUG -O2 -D_FORTIFY_SOURCE=2 $(LDFLAGS)
 CFLAGS += $(CFG)
 
 LIBS += $(shell pkg-config --libs $(PACKAGES))
 INCS += $(shell pkg-config --cflags $(PACKAGES))
+
 CFLAGS += -Wall -std=c99
 OBJS = compton.o
 
+# === Recipes ===
+.DEFAULT_GOAL := compton
+
 %.o: src/%.c src/%.h
 	$(CC) $(CFLAGS) $(INCS) -c src/$*.c
 
@@ -75,4 +113,4 @@ uninstall:
 clean:
 	@rm -f $(OBJS) compton
 
-.PHONY: uninstall clean
+.PHONY: uninstall clean docs