install newly required header in autotools build system

sass_interface.h #includes sass2scss.h so we need to have the
latter header wherever we have the former one.
Merge pull request #609 from idupree/autotools-header-install-fix

install newly required header in autotools build system
Fixes and improves travis ci build

Changes -std=c++11 to -std=c++0x!

Conflicts:
	Makefile.am
Aligns build system with autotools build and travis-ci

Implements BUILD variable to link shared or static
Expects libraries in lib directory as with autotools build