Expand use of pkg-config variables.

Change pkg-config generation such that the path variables,
not their values, are used in the definitions of Libs and Cflags,
and that $prefix is substituted into libdir and includedir iff
they start with its value.

This makes it easier to modify the already installed file if necessary.

lib/Makefile

@@ -175,6 +175,7 @@ liblz4.pc: liblz4.pc.in Makefile
          -e 's|@LIBDIR@|$(libdir)|' \
          -e 's|@INCLUDEDIR@|$(includedir)|' \
          -e 's|@VERSION@|$(LIBVER)|' \
+         -e 's|=${prefix}/|=$${prefix}/|' \
           $< >$@
 
 install: lib liblz4.pc

lib/liblz4.pc.in

@@ -10,5 +10,5 @@ Name: lz4
 Description: extremely fast lossless compression algorithm library
 URL: http://www.lz4.org/
 Version: @VERSION@
-Libs: -L@LIBDIR@ -llz4
-Cflags: -I@INCLUDEDIR@
+Libs: -L${libdir} -llz4
+Cflags: -I${includedir}