Respect user's pkg-config
--- a/Makefile
+++ b/Makefile
@@ -6,13 +6,13 @@ LOCALEDIR ?= /usr/local/share/locale
 CFLAGS += -DPACKAGE="\"$(PACKAGE)\"" 
 export PACKAGE LOCALEDIR
 
-GTK_CONFIG = pkg-config gtk+-2.0
+PKG_CONFIG ?= pkg-config
 
 PLUGIN_DIR ?= /usr/local/lib/gkrellm2/plugins
 GKRELLM_INCLUDE = -I/usr/local/include
 
-GTK_CFLAGS = `$(GTK_CONFIG) --cflags` 
-GTK_LIB = `$(GTK_CONFIG) --libs`
+GTK_CFLAGS = $(shell ${PKG_CONFIG} gtk+-2.0 --cflags)
+GTK_LIB = $(shell ${PKG_CONFIG} gtk+-2.0 --libs)
 
 CFLAGS := $(CFLAGS) -fPIC $(GTK_CFLAGS) $(GKRELLM_INCLUDE)
 LIBS = $(GTK_LIB)
@@ -51,4 +51,4 @@ install:
 	(cd po && ${MAKE} install)
 	$(INSTALL_PROGRAM) volume.so $(PLUGIN_DIR)
 
-%.c.o: %.c
\ No newline at end of file
+%.c.o: %.c