Allow passing --with-openssl-dir for a custom OpenSSL installation

[kalbasit]

OpenSSL isn't located on standard paths on my Linux machine and thus I can't get Semian gem to install.

Before the flag

❯ ruby ext/semian/extconf.rb
checking for openssl/sha.h... no
openssl is missing. please install openssl.
*** ext/semian/extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of necessary
libraries and/or headers.  Check the mkmf.log file for more details.  You may
need configuration options.

Provided configuration options:
        --with-opt-dir
        --without-opt-dir
        --with-opt-include
        --without-opt-include=${opt-dir}/include
        --with-opt-lib
        --without-opt-lib=${opt-dir}/lib
        --with-make-prog
        --without-make-prog
        --srcdir=ext/semian
        --curdir
        --ruby=/nix/store/3ay21xsh45kjqfpsan7xmnf3mbng29d7-ruby-3.0.2/bin/$(RUBY_BASE_NAME)

After the flag

❯ ruby ext/semian/extconf.rb --with-openssl-dir=$(readlink -f result)
checking for openssl/sha.h... yes
checking for SHA1() in -lcrypto... yes
checking for sys/ipc.h... yes
checking for sys/sem.h... yes
checking for sys/types.h... yes
checking for rb_thread_blocking_region()... no
checking for rb_thread_call_without_gvl()... yes
creating Makefile

[kalbasit]

I have signed the CLA!

[kalbasit]

@casperisfine mind review the pr?

[casperisfine]

Thanks