Improvement: Support reading _NET_WM_OPACITY from client windows

- Some WMs don't respect Keith Packard's proposal of
  _NET_WM_WINDOW_OPACITY, and do not copy _NET_WM_OPACITY attribute of a
  client window to its frame windows, thus cause opacity set by
  non-override-redirect windows to have no effect. This commit adds
  support for reading _NET_WM_OPACITY from client windows if running
  with --detect-client-opacity. Thanks to pvanek for reporting.

- Change Makefile logic to determine options from 3 variables
  (NO_LIBCONFIG, NO_REGEX_PCRE, NO_REGEX_PCRE_JIT) instead of CFG to
  ensure compatibility when we add new options. CFG variable is no
  longer been respected.

Makefile

@@ -4,20 +4,38 @@ PREFIX ?= /usr
 BINDIR ?= $(PREFIX)/bin
 MANDIR ?= $(PREFIX)/share/man/man1
 
-PACKAGES = x11 xcomposite xfixes xdamage xrender xext libconfig
-LIBS = $(shell pkg-config --libs $(PACKAGES)) -lm
-LIBS += $(shell pcre-config --libs)
-INCS = $(shell pkg-config --cflags $(PACKAGES))
-INCS += $(shell pcre-config --cflags)
+PACKAGES = x11 xcomposite xfixes xdamage xrender xext
+LIBS = -lm
+INCS =
+
+# Parse configuration flags
+CFG =
+
+ifeq "$(NO_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 += $(shell pkg-config --atleast-version=1.4 libconfig || echo '-DCONFIG_LIBCONFIG_LEGACY')
+endif
+
+ifeq "$(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
+endif
+
+CFLAGS += $(CFG)
+
+LIBS += $(shell pkg-config --libs $(PACKAGES))
+INCS += $(shell pkg-config --cflags $(PACKAGES))
 CFLAGS += -Wall -std=c99
 OBJS = compton.o
 
-CFG ?= -DCONFIG_LIBCONFIG -DCONFIG_REGEX_PCRE -DCONFIG_REGEX_PCRE_JIT
-# 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')
-CFLAGS += $(CFG)
-
 %.o: src/%.c src/%.h
 	$(CC) $(CFLAGS) $(INCS) -c src/$*.c