Same issue as seen before with libssh2: `libpsl`'s pkg-config module
depends on another module, but that's not found. CMake ends up reporting
`LIBPSL_FOUND=YES`, while leaving `LIBPSL_INCLUDE_DIRS` empty. Then
the build fails to find `psl.h`.
The missing dependency in this case is `icu4c`, which is "keg-only",
meaning it's not exposed in the default Homebrew header, pkg-config,
lib, etc locations. It must be added to the `PKG_CONFIG_PATH` env, as
suggested by the warnings messages of `pkgconf`.
To avoid this fallout, let's ensure that `LIBPSL_INCLUDE_DIRS` is
non-empty when detecting via `pkg-config` and fall back to the CMake
detection method otherwise.
This was an issue till Homebrew libpsl 0.21.5_1, fixed in 0.21.5_2, that
no longer depends on `icu4c`.
Example log:
```
-- Checking for module 'libpsl'
-- Found libpsl, version 0.21.5
Package icu-uc was not found in the pkg-config search path.
Perhaps you should add the directory containing `icu-uc.pc'
to the PKG_CONFIG_PATH environment variable
Package 'icu-uc', required by 'libpsl', not found
[...]
-- Found Libpsl (via pkg-config): (found version "0.21.5")
[...]
In file included from curl/_bld/lib/CMakeFiles/libcurl_static.dir/Unity/unity_0_c.c:4:
In file included from curl/lib/altsvc.c:32:
In file included from curl/lib/urldata.h:145:
curl/lib/psl.h:28:10: fatal error: 'libpsl.h' file not found
^~~~~~~~~~
1 error generated.
```
Follow-up to 39c741b7b0#15408Closes#15827
98f419172b