pkgconf: upgrade 2.2.0 -> 2.3.0

pkgconf: upgrade 2.2.0 -> 2.3.0

Changelog:
==========
* Fix compile with Meson on Solaris by defining __EXTENSIONS__.
* Add support for the PKG_CONFIG_RELOCATE_PATHS environmental variable.
  When set, the program will act as if --define-prefix is always
  enabled.
* Color solution nodes that were part of the original query, and use
  that coloring to skip over dependencies when generating DocumentNames
  in bomtool.
* Enhance --env option to support variables with both --variable=varname
  and --print-variables.
* Add --exists-cflags option which creates synthetic preprocessor
  definition flags for every queried dependency when found.
* Document that Requires.private is always used for header paths.
* Fix minor documentation typos.
* Ensure string comparisons using <ctype.h> functions are done with
  unsigned bytes to avoid undefined behavior.
* Fix parsing edge-case bugs with dependency versions.
* Change PKG_PROG_PKG_CONFIG autoconf macro to add a customizable
  failure handler if pkg-config is not found.

Signed-off-by: Wang Mingyu <wangmy@fujitsu.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>