Pkgconf

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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)

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* Bug fixes:
- rewrite handling of --modversion, --print-variables and --variable to not require
the dependency resolver
- ensure we disable the dependency resolver in all cases where it is a single-package
query (1.3.1 did not go far enough)