Add VHDL Library support to GHDL Synthesis
As per issue #115. This merge request is to merge code that allows HDLmake to give GHDL Synthesis a correct set of files to support VHDL libraries.
This should be merged into develop if the branch ltgt_bug_#113 is already merged.
There are 2 commits in the branch, the first adds a testcase that demonstrates what we need to get the code to be accepted by GHDL_synthesis. The second commit contains the change to hdlmake that will allow it to create the correct Makefile for library support and a fix for a missing default_library attribute.