Make pkgconfig-depends usable

[bgamari]

Describe the bug
Cabal has for a long time supported discovering native library dependencies through pkgconfig-depends. This is extremely useful as:

• it can be used by some distributions to transparently provide native dependencies to cabal-install (e.g. see nix-pkgconfig)
• it makes Cabal packages more robust against non-traditional packaging decisions
• it eliminates the need for Cabal to guess whether the target platform has a particular library

However, due to the fact that pkg-config is not available by default on some platforms, libraries tend to either not use pkgconfig-depends (e.g. see regex-posix) or guard it behind a default-off flag (e.g. see zlib). This severely cripples the usefulness of the mechanism.

Even worse, the tendency for packages to assume library non-availability on Windows can blow up catastrophically in some cases. This was the case in GHC #19945 where an ABI mismatch triggered by the assumption that Windows environments do not offer a POSIX regex implementation caused an innocuous-looking program to segfault, requiring many hours of debugging.

Proposed design

Users avoid using pkgconfig-depends by default because it can cause builds to fail when pkg-config is not available. I suggest one of the following (roughly in order of decreasing desirability):

• cabal-install learn to backtrack if a pkg-config probe fails due to the executable not being available
• cabal-install implement support for setting/clearing the pkgconfig flag on its own depending upon whether the pkg-config executable is available, sanctioning the current flag-based convention
• cabal-install assume that the pkgconfig-depends are available in the event that pkg-config is not available

[phadej]

There are related issues:

• Allowing pkg-config to fail #6771 (Allowing pkg-config to fail), mentions has(pkg-config) conditional
• Extend the grammar/semantics of default: to support boolean expressions #5328 Extend the grammar/semantics of default: to support boolean expressions - with has(pkg-config would allow setting flags based on existence of pkg-config.

If these are implemented, then both first and second @bgamari suggestions will be possible.

[gbaz]

I think I can have a go at option 1. Would appreciate someone pointing me to the simplest possible package that uses pkgconfig-depends they can think of so I can use it as a test case :-)

(And ideally this would have a cabal file example of how to build it in the no pkg-config case too!)

[phadej]

About any, e.g. https://hackage.haskell.org/package/zlib-0.6.2.3/zlib.cabal has manual flag to select pkg-config, while on my system:

% pkg-config --list-all|grep zlib
zlib                                zlib - zlib compression library

(I forgot to install zlib1g-dev in CI environments so many times, I remember that by heart). lzma is another.

https://hackage.haskell.org/package/HsOpenSSL is more complicated, where you start to wish pkg-config told things to avoid:

    if os(darwin) && flag(homebrew-openssl)
        Include-Dirs: /usr/local/opt/openssl/include
        Extra-Lib-Dirs: /usr/local/opt/openssl/lib

    if os(darwin) && flag(macports-openssl)
        Include-Dirs: /opt/local/include
        Extra-Lib-Dirs: /opt/local/lib