Aligns build system with autotools build and travis-ci

This PR is linked to https://github.com/sass/libsass/pull/606
Will not pass travis ci until both PRs are merged!

Improves the build system to support shared object linking.
Drops the need for newer gcc as it now uses `-std=c++0x`

Implements BUILD variable to link shared or static objects.
Expects libraries in lib directory as with autotools build.

https://travis-ci.org/mgreter/libsass/builds/39788451

Thanks to @QuLogic for his reviews in https://github.com/sass/libsass/pull/606