Use PKG_CHECK_MODULES() for calling and overriding pkg-config

- macro is in build/pkg.m4 from pkg-config 0.28
- Use <PREFIX>_CFLAGS and <PREFIX>_LIBS envars.
- Enables overriding of pkg-config if needed.

Patch from Michael Stahl

Fixes Issue #0000541
http://bugs.librdf.org/mantis/view.php?id=541
Remove SQLITE V2 support and switch to PKG_CHECK_MODULES in configure

- Remove ancient sqlite V2 API support, removing lots of conditional code.
- Alter configure to use PKG_CHECK_MODULES for sqlite3 and then the
  SQLITE_CFLAGS and SQLITE_LIBS envars