Have Makefile respect user-configured pkg-config
--- a/Makefile
+++ b/Makefile
@@ -1,10 +1,10 @@
 # Makefile for the GKrellM mailwatch plugin
 
-GTK_CONFIG ?=pkg-config gtk+-2.0
+PKG_CONFIG ?= pkg-config
 PLUGIN_DIR ?=/usr/local/lib/gkrellm2/plugins
 
-GTK_INCLUDE = `$(GTK_CONFIG) --cflags`
-GTK_LIB = `$(GTK_CONFIG) --libs`
+GTK_INCLUDE = $(shell ${PKG_CONFIG} gtk+-2.0 --cflags)
+GTK_LIB = $(shell ${PKG_CONFIG} gtk+-2.0 --libs)
 
 GKRELLM_INCLUDE= -I/usr/local/include