Detect pkgconf on Windows/mingw

[mtelvers]

Using the Opam 2.2.0 release candidate 1 to install sqlite3-ocaml on Windows I ran into this problem

$ dune build
File "src/dune", lines 10-12, characters 0-93:
10 | (rule
11 |   (targets c_flags.sexp c_library_flags.sexp)
12 |   (action (run ./config/discover.exe)))
'pkg-config' is not recognized as an internal or external command,
operable program or batch file.
read_lines_from_cmd: failed reading line 1, cmd: ' pkg-config --cflags sqlite3'Fatal error: exception End_of_file

The compiler flags can be obtained by running, for example:

$ pkgconf --personality=x86_64-w64-mingw32 --libs sqlite3
-lsqlite3

This PR updates discover.ml to use this syntax on Windows.

[shonfeder]

LGTM, fwiw! :)

Being able to merger and release this would let us unblock ocaml/opam-repository#26146 . Let me know if there is anything I could do to support, @mmottl!

[mmottl]

Thanks for the contribution!