cmake: add option to build and link BoringSSL

[angt]

No description provided.

[ggerganov]

This does not build on Mac:

cmake -B build-boring -DLLAMA_BUILD_BORINGSSL=ON
cmake --build build-boring -j

[100%] Building CXX object tools/server/CMakeFiles/llama-server.dir/server.cpp.o
In file included from /Users/ggerganov/development/github/llama.cpp/tools/server/server.cpp:2:
In file included from /Users/ggerganov/development/github/llama.cpp/tools/server/utils.hpp:12:
/Users/ggerganov/development/github/llama.cpp/common/../vendor/cpp-httplib/httplib.h:340:10: fatal error: 'openssl/err.h' file not found
  340 | #include <openssl/err.h>
      |          ^~~~~~~~~~~~~~~
1 error generated.
make[2]: *** [tools/server/CMakeFiles/llama-server.dir/server.cpp.o] Error 1
make[1]: *** [tools/server/CMakeFiles/llama-server.dir/all] Error 2

[angt]

It worked for me because I tested OpenSSL just before I guess..

[angt]

Fixed, I thought I could put the library in PRIVATE but the header still exposes all the SSL stuff.