Feature: OpenGL backend

- Add experimental OpenGL backend (--opengl). --blur-background is
  currently not possible with this backend, because I'm still trying to
  find a proper way to do blur with OpenGL. Flipping backend on-the-fly
  is really hard, so it isn't supported right now. No configuration file
  option exists to enable this, because it isn't stable enough.

- Add `opengl-swc` VSync method that uses SGI_swap_control to control
  buffer swap, with OpenGL backend. (#7)

- Fix a potential read-from-freed-memory issue in paint_all().

- Correctly reattach GLX context after fork.

- Dump error text in error(). Add GLX error code handling.

- Code clean-up.

- Known issues: Region operations take a lot of time in glx_render().
  I'm hesitating about what to do.

Makefile

@@ -47,6 +47,7 @@ ifeq "$(NO_VSYNC_OPENGL)" ""
   CFG += -DCONFIG_VSYNC_OPENGL
   # -lGL must precede some other libraries, or it segfaults on FreeBSD (#74)
   LIBS := -lGL $(LIBS)
+  OBJS += opengl.o
 endif
 
 # ==== D-Bus ====
@@ -67,7 +68,16 @@ COMPTON_VERSION ?= git-$(shell git describe --always --dirty)-$(shell git log -1
 CFG += -DCOMPTON_VERSION="\"$(COMPTON_VERSION)\""
 
 LDFLAGS ?= -Wl,-O1 -Wl,--as-needed
-CFLAGS ?= -DNDEBUG -O2 -D_FORTIFY_SOURCE=2
+
+ifeq "$(DEV)" ""
+  CFLAGS ?= -DNDEBUG -O2 -D_FORTIFY_SOURCE=2
+else
+  CC = clang
+  export LD_ALTEXEC = /usr/bin/ld.gold
+  OBJS += backtrace-symbols.o
+  LIBS += -lbfd
+  CFLAGS += -ggdb -Weverything -Wno-disabled-macro-expansion -Wno-padded -Wno-gnu
+endif
 
 LIBS += $(shell pkg-config --libs $(PACKAGES))
 INCS += $(shell pkg-config --cflags $(PACKAGES))