cmake: fix for building with no internet connection

[Man2Dev]

This PR will ensure that the test for curl integration in llama.cpp is only conducted if an internet connection is available. If no internet is available, it
will simply provide a warning and not test the curl integration.

The reason for this PR is to allow all other tests to be used in environments where internet access is not available at build time without causing the tests to
fail. For example, in Fedora/RedHat package build systems like mock [1], where no internet is provided during the build process.

[1] https://rpm-software-management.github.io/mock/

[ggerganov]

This is not a good way to fix this. Instead, there should be an option to filter tests that require internet connection and this option can be optionally set during CMake configure step.