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/Makefile

@@ -244,8 +244,6 @@ libzstd.pc:
 libzstd.pc: libzstd.pc.in
 	@echo creating pkgconfig
 	@sed -e 's|@PREFIX@|$(PREFIX)|' \
-             -e 's|@LIBDIR@|$(LIBDIR)|' \
-             -e 's|@INCLUDEDIR@|$(INCLUDEDIR)|' \
              -e 's|@VERSION@|$(VERSION)|' \
              $< >$@