[GNC] Building GC 5.4 on Ubuntu 20.04

[Tim Kallmer]

Is it not possible to build GC 5.4 on Ubuntu 20.04?

I installed all the build requirements, including gwenhywfar-tools. It
looks like Ubuntu 20.04 will only install gwenhywfar 5.1.3. So during make,
it is looking for gwenhywfar 5.6.0. and then it fails as below:

-- Checking for module 'gwenhywfar>=5.6.0'
--   Requested 'gwenhywfar >= 5.6.0' but version of gwenhywfar is 5.1.3
CMake Error at /usr/share/cmake-3.16/Modules/FindPkgConfig.cmake:463
(message):
  A required package was not found

If it's not possible, I am fine staying at GC 4.9.