Add VHDL Library support to GHDL Synthesis

See merge request !8

Add VHDL Library support to ISE

See merge request !7

Simplify the code.
Adjust baselines

Fix #112

the default library on Xilinx is xil_default, which is also the library
where XCI designs are put.
If the default library is 'work', then an instantiation like 'entity work.ip'
will look in library 'work' which doesn't contain XCI designs.

Adjust baselines

Adjust baseline

extract the provided unit

clarify code.

Fix #111