Switch to using pkg-config

[AdrianFreundQC]

Checklist

• Used a personal fork of the feedstock to propose changes
• Bumped the build number (if the version is unchanged)
• Reset the build number to 0 (if the version changed)
• Re-rendered with the latest conda-smithy (Use the phrase @conda-forge-admin, please rerender in a comment in this PR for automated rerendering)
• Ensured the license file is being packaged.

[conda-forge-webservices]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe) and found it was in an excellent condition.

[AdrianFreundQC]

Next attempt after leaving this change out of #20.

Cross-compiling fails with

run pkg_config fail: pkg-config has not been configured to support cross-compilation.

Install a sysroot for the target platform and configure it via
PKG_CONFIG_SYSROOT_DIR and PKG_CONFIG_PATH, or install a
cross-compiling wrapper for pkg-config and set it via
PKG_CONFIG environment variable.

Manually setting PKG_CONFIG_SYSROOT_DIR makes pkg-config find the library, but later causes linking to fail.

Note that in this feedstock it only fails on Linux, as the MacOS build doesn't link against OpenSSL, but MacOS's native SSL library, which doesn't require using pkg-config.

Also see the two different workarounds at conda-forge/cargo-bundle-licenses-feedstock#8 and conda-forge/cargo-c-feedstock#1, the bug report at conda-forge/rust-activation-feedstock#35 as well as the documentation for rust's pkg-config crate: https://docs.rs/pkg-config/latest/pkg_config/.

This issue should ideally be solved in rust-activation-feedstock so different feedstocks don't need to copy-and-paste the same build scripts over and over.

CC: @pavelzw @xhochy