Respect user's pkg-config
--- a/Makefile
+++ b/Makefile
@@ -1,5 +1,6 @@
-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 = $(shel ${PKG_CONFIG} gtk+-2.0 --libs)
 
 INSTALLDIR = ${DESTDIR}/usr/lib/gkrellm2/plugins