Use pkg-config for determining the OpenSSL path instead of using /usr/local by default under Linux

[moritzheiber]

Pretty much any modern Linux distribution is using pkg-config these days to share a few common details about the way packages are configured and installed on the host system. The openssl package (at least under Ubuntu/Debian) exposes the prefix it is installed under with the prefix variable in pkg-config.

These modifications are using pkg-config to determine the prefix for openssl.

Using /usr/local as a default will fail to build Ruby with OpenSSL support, "silently" (it's just not detected at compile time), with the symptom simply being that any Ruby built with frum won't have TLS/SSL support. This is especially relevant for the latest Ubuntu release (22.04, Jammy Jellyfish) since it ships with OpenSSL 3.x for which there currently is no support in the OpenSSL gem.