Remove parsing of the X11LIB variable

X11LIB was introduced along with other similar variables such as X11INC
in 1faee75, before dunst used pkg-config to manage libraries. These
variables were removed after the introduction of pkg-config in 0de2e17
so theoretically it is safe to remove it.

I really hope this doesn't break something...

config.mk

@@ -15,7 +15,7 @@ PKG_CONFIG ?= pkg-config
 # flags
 CPPFLAGS += -D_DEFAULT_SOURCE -DVERSION=\"${VERSION}\"
 CFLAGS   += -g --std=gnu99 -pedantic -Wall -Wno-overlength-strings -Os ${STATIC} ${CPPFLAGS}
-LDFLAGS  += -lm -L${X11LIB}
+LDFLAGS  += -lm
 
 CPPFLAGS_DEBUG := -DDEBUG_BUILD
 CFLAGS_DEBUG   := -O0