Allow running pkg-config when targetting windows from non-windows hosts

[micolous]

FIxes #1984

Unfortunately, the cross-compile tests in CI don't actually use pkgconfig at all, because they set OPENSSL_DIR:

https://github.com/sfackler/rust-openssl/blob/0f0bbe42bcfe817bb943e3ed36efee9a875c8458/.github/workflows/ci.yml#L209

Testing on a macOS host with PKG_CONFIG_PATH_x86_64_pc_windows_gnu pointing at a vcpkg-installed version of OpenSSL:

% CARGO_TARGET_X86_64_PC_WINDOWS_GNU_RUNNER=wine64 cargo test --target x86_64-pc-windows-gnu         
   Compiling openssl-sys v0.9.90 (~/rust-openssl/openssl-sys)
   Compiling openssl v0.10.55 (~/rust-openssl/openssl)
    Finished test [unoptimized + debuginfo] target(s) in 6.92s
     Running unittests src/lib.rs (~/rust-openssl/target/x86_64-pc-windows-gnu/debug/deps/openssl-48026ed6f097cb90.exe)
preloader: Warning: failed to reserve range 0000000000010000-0000000000110000
preloader: Warning: failed to reserve range 0000000000010000-0000000000110000
preloader: Warning: failed to reserve range 0000000000010000-0000000000110000
preloader: Warning: failed to reserve range 0000000000010000-0000000000110000
preloader: Warning: failed to reserve range 0000000000010000-0000000000110000
preloader: Warning: failed to reserve range 0000000000010000-0000000000110000
007c:fixme:hid:handle_IRP_MN_QUERY_ID Unhandled type 00000005
007c:fixme:hid:handle_IRP_MN_QUERY_ID Unhandled type 00000005
007c:fixme:hid:handle_IRP_MN_QUERY_ID Unhandled type 00000005
preloader: Warning: failed to reserve range 0000000000010000-0000000000110000
preloader: Warning: failed to reserve range 0000000000010000-0000000000110000
007c:fixme:hid:handle_IRP_MN_QUERY_ID Unhandled type 00000005
preloader: Warning: failed to reserve range 0000000000010000-0000000000110000
preloader: Warning: failed to reserve range 0000000000010000-0000000000110000
preloader: Warning: failed to reserve range 0000000000010000-0000000000110000

running 370 tests
[snip]

test result: ok. 364 passed; 0 failed; 6 ignored; 0 measured; 0 filtered out; finished in 3.50s

[sfackler]

Thanks!

[sfackler]

I realized right after merging that we'll want slightly different logic but it should still cover your use case: 4d2379f

[micolous]

Yeah, that'll work too, thanks!