vendor: split httplib to cpp/h files

[ngxson]

Split httplib.h into cpp and header files, allowing it to be compiled as separated unit

master:
cmake --build build -j --target llama-server  24.28s user 1.53s system 99% cpu 25.958 total

PR:
cmake --build build -j --target llama-server  16.38s user 0.62s system 100% cpu 16.963 total

Ref script: https://github.com/yhirose/cpp-httplib/blob/master/httplib.h

[ggerganov]

Not sure why this is needed - seems incorrect.

[ngxson]

The line next to this suggests that it's used by httplib, CPPHTTPLIB_OPENSSL_SUPPORT

So I assume that it need to be present here. Seems to be used by the download functionality (alternative to curl), right?

[ngxson]

Otherwise, I think it should be in the if NOT LLAMA_CURL condition, meaning if we don't use curl, then we will use httplib. Though I'm not sure if this is correct. CC @angt

[ngxson]

Ah nevermind, it's correct that libcommon needs cpp-httplib if CURL is not used. Last commit should fix the CI error: https://github.com/ggml-org/llama.cpp/actions/runs/19241641967/job/55005558638?pr=17150