pkg-config search path

Hi,

Cabal doesn't honour PKG_CONFIG_PATH, which can be updated after package install to expose it.

It prevents resolving pkg-config dependencies (whereas the same deps are found executing pkg-config by hand on CLI).

Env: Win7 msys2 cabal-1.18

No file or test is named in the report. Start by reproducing the dependency-resolution difference on Windows/MSYS2 with Cabal 1.18, comparing Cabal's behavior with running pkg-config directly and checking PKG_CONFIG_PATH. Done means dependencies exposed through that variable resolve consistently with the command-line tool.

Written by the indexing model from the issue text.