Set the path of pkg-config in a variable instead of hardcoding it

In this way the path of pkg-config can be overridden from the command
line. This is useful for example when cross-compiling.

config.mk

@@ -10,13 +10,15 @@ MANPREFIX = $(PREFIX)/share/man
 X11INC = /usr/X11R6/include
 X11LIB = /usr/X11R6/lib
 
+PKG_CONFIG = pkg-config
+
 # includes and libs
 INCS = -I$(X11INC) \
-       `pkg-config --cflags fontconfig` \
-       `pkg-config --cflags freetype2`
+       `$(PKG_CONFIG) --cflags fontconfig` \
+       `$(PKG_CONFIG) --cflags freetype2`
 LIBS = -L$(X11LIB) -lm -lrt -lX11 -lutil -lXft \
-       `pkg-config --libs fontconfig` \
-       `pkg-config --libs freetype2`
+       `$(PKG_CONFIG) --libs fontconfig` \
+       `$(PKG_CONFIG) --libs freetype2`
 
 # flags
 CPPFLAGS = -DVERSION=\"$(VERSION)\" -D_XOPEN_SOURCE=600