Can't install Ruby with MacPorts and LibreSSL

[Robiathin]

Description

When using MacPorts (OSX), if LibreSSL is installed then rvm will fail to install a Ruby. Since LibreSSL is a fork of OpenSSL, it can be used instead. I have written a fix for this issue myself and I can open a pull request.

Steps to reproduce

1. port install libressl
2. rvm install 2.5

Expected behavior

If LibreSSL is installed, don't try and install OpenSSL when using MacPorts.

Actual behavior

rvm install errors out.

Searching for binary rubies, this might take some time.
No binary rubies available for: osx/10.12/x86_64/ruby-2.5.
Continuing with compilation. Please read 'rvm help mount' to get more information on binary rubies.
Checking requirements for osx.
Installing requirements for osx.
Updating system..rob password required for 'port -dv selfupdate': 
........
Installing required packages: openssl...There were package installation errors, make sure to read the log.
.
Error running 'requirements_osx_port_libs_install openssl',
please read /Users/rob/.rvm/log/1508360781_ruby-2.5/package_install_openssl.log
Requirements installation failed with status: 1.