Instead of hardcoding dependencies, detect and use what is found

Currently extconf.rb just hardcodes the build flags. This works for Linuxes and Macs, and on Windows/Mingw, but does not work with Visual C++. This changeset adds detection of available libraries and compatibility with the Visual C++ Toolchain. 

This should also simplify building on Windows in general since OpenSSL uses ssleay32 and libeay32 as the library name.

Tested on my Visual C++ 2013 build. Should work with RubyInstaller 2.0.0 also.