ghdl 0.18-2

[elhigu]

1. version ghdl 0.20 is new enough

2. Breezy already has ghdl 0.18 so compiling version 0.20 should not need
any other updates to system.

3. current version 0.18 doesn't support -fexplicit compile flag that makes
it possible to use synopsys and other ieee extensions.

[elhigu]

Actually ghdl 0.21 from sid uses newer version of libgnat, but breezy already
contains libgnat-4.0.

[elhigu]

ubp-build.py script did generate working ghdl-0.21-1 package from Sid sources.