Use RbConfig::MAKEFILE_CONFIG['DLEXT'] instead of hardcoding extensions
* Since libsass is meant to be used with FFI, RbConfig::CONFIG['SOEXT']
would be better, but DLEXT is used as libsass is compiled by mkmf (#127).
* Use RbConfig::MAKEFILE_CONFIG['DLEXT'] instead of just RbConfig::CONFIG['DLEXT']
so it's also correct on JRuby and not 'jar'. See
* This lets Ruby implementations use another file extension than .so/.bundle
for C extensions. For example, TruffleRuby 19.3.0 uses .dylib on macOS.
* Some OS also use a different file extension, such as HPUX which uses .sl.