Curl has an option to use libidn2 in version 8.9.0.

Pkg-config may unexpectedly find it and use it.

I encountered this on Windows because libidn2 was installed by
Strawberry Perl, under C:\Strawberry. Pkg-config found that and tried to
build it into Curl, which is problematic in part because we don't want
to include or ship libidn2 with our software.

But also problematic because libidn2 provided by Strawberry Perl used a
different OpenSSL incompatible with the one we're compiling against.

This commit explicitly disables pkg-config and libidn2 for libcurl
builds.
TODO: Perhaps disable pkg-config for all recipes and all platforms?
Mussels builds should only draw from other mussels include/lib files.

nghttp2 -> 1.63.0

libxml2 -> 2.13.3

Because of possible linking path issues

Xcode dropped support for 'buildsystem=1' option.

Disable building docs on unix systems and ensure only building library.
The docs build is what was failing the Xcode "new build system" build.

Automatic library detection finds brew-installed libs.
This is primarily to fix builds on GitHub Actions where
the system provides all sorts of stuff we didn't request.

On macOS, specifically, this causes failures because we build
universal binaries, and brew binaries are not universal and
lack symbols for othe other platform.

It seems like we're detecting system-provided headers.

If the system headers differ from our lib, we may end up with 'missing' symbols.

This is to fix a build issue on macOS in GitHub Actions.