Have Makefile respect user chosen PKG_CONFIG
--- a/Makefile
+++ b/Makefile
@@ -3,8 +3,10 @@
 # You may want to rename the binary-file.
 BIN_FILENAME = gkrellm-countdown
 
-GTK_INCLUDE ?= `pkg-config gtk+-2.0 --cflags`
-GTK_LIB ?= `pkg-config gtk+-2.0 --libs`
+PKG_CONFIG ?= pkg-config
+
+GTK_INCLUDE ?= $(shell ${PKG_CONFIG} gtk+-2.0 --cflags)
+GTK_LIB ?= $(shell ${PKG_CONFIG} gtk+-2.0 --libs)
 IMLIB_INCLUDE ?= 
 IMLIB_LIB ?= 
 PLUGIN_DIR ?= /usr/local/lib/gkrellm2/plugins