Add option to control whether to use `pkg-config` to detect
dependencies. Curl's CMake uses `pkg-config` by default for all targets
except for MSVC without vcpkg.
With the CMake option `-DCURL_USE_PKGCONFIG=ON` you can override it to
use `pkg-config` always.
If `pkg-config` is causing issues, e.g. in cross-builds or other cases,
`-DCURL_USE_PKGCONFIG=OFF` disables all use of `pkg-config`.
Also add it to `curl-config.cmake`. Not yet used, but will be once curl
starts referencing any curl-specific `Find*` module from this public
script.
Follow-up to 9dfdc6ff42#14483Closes#14504
b910122fe3