Pkgconf

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* Notable libpkgconf API changes:
- pkgconf_pkg_t.requires has been renamed to pkgconf_pkg_t.required for
C++20 compatibility.

* Enhancements:
- pkgconf and libpkgconf has been ported to Windows as native binaries.
- improved compatibility with freedesktop.org pkg-config's ${pc_sysrootdir}
usage pattern.
- do not mention PKG_CONFIG_SKIP_CONFLICTS environmental variable when
simplified errors are requested, as with PKG_CONFIG_PATH.
- the dependency solver now stores solutions to dependency graph elements
it visits, allowing for the dependency graph to be incrementally solved.
this improves dependency solving time by an order of magnitude in most
cases.
- new --env option allows for exporting cflags/libs fragments as export
variables
- new support for building pkgconf with CMake and Meson
- improved compiler warning flag detection on autoconf and CMake
- removed PKGCONF_BUFSIZE allocations from the stack where possible
- allow for customizing the way fragment lists are rendered using a callback API
- new support for --msvc-syntax output using the new fragment rendering callbacks
- fragments are now quoted according to POSIX literal rules
- new variables on the pkg-config builtin:
- ${pc_system_includedirs}: the system includedir search path known by pkgconf
- ${pc_system_libdirs}: the system libdir search path known by pkgconf
- new manpages:
- pc(5) describing pkgconf's interpretation of pkg-config .pc files
- pkg.m4(7) describing the autotools macros bundled with pkgconf

* Bug fixes:
- fix pkgconf_pkg_t.id generation on native Windows where either \ or / are usable
as path separator.
- add missing --modversion to --help output
- do not evaluate module paths for modules that are not actually on disk
- ensure we work on a zeroed buffer prior to calling realpath(2) with it
- fix path deduplication edge case when cache-inodes feature is unavailable
- fix path rewriting regression with PKG_CONFIG_SYSROOT_DIR when
PKG_CONFIG_SYSROOT_DIR is set to /
- fix crash in edge case where a .pc file has misquoting in a fragment list.
- fix logic edge case when comparing relocated paths

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* Enhancements:
- improved diagnostics for malformed packages.

* Bug fixes:
- reject packages which contain incomplete metadata in post-parse phase.

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* Enhancements:
- add many cflags to the protected set: -Wa, -Wl, -Wp, -ansi, -std=, -stdlib=,
-pedantic, -pthread, -trigraphs, -nostdinc, -nostdlibinc, -nobuiltininc.

* Bug fixes:
- handle -include cflag fragments properly.

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* Bug fixes:
- fix --variable output for compatibility some broken configure scripts when they
request the same variable from multiple packages

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* Bug fixes:
- fix a quoting issue exposed by the Go testsuite

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* Bug fixes:
- back out disabling the dependency resolver for single-package queries, it caused
too many regressions.

* Enhancements:
- allow explicitly disabling the dependency resolver via new environment variable,
PKG_CONFIG_MINIMUM_TRAVERSE_DEPTH=1. while pkgconf could already do this using
--minimum-traverse-depth=1, other pkg-config implementations do not have this
option, so adding an environment variable allows to make better use of this
feature (other implementations won't error due to unknown option this way)