In pkg-config File, Derive Lib and Include Dir from Prefix at Use-Time

Addresses #1794. Instead of deriving the lib dir and include dir at
build-time, let's do it like everyone else does at pkg-config run-time.

This has the disadvantage that we can no longer override LIBDIR and
INCLUDEDIR in the Makefile and have that reflected in the .pc file.

lib/libzstd.pc.in

@@ -3,8 +3,9 @@
 #   BSD 2-Clause License (http://www.opensource.org/licenses/bsd-license.php)
 
 prefix=@PREFIX@
-libdir=@LIBDIR@
-includedir=@INCLUDEDIR@
+exec_prefix=${prefix}
+includedir=${prefix}/include
+libdir=${exec_prefix}/lib
 
 Name: zstd
 Description: fast lossless compression algorithm library