Compilation on Ubuntu 20.04 LTS

[sheikki]

In order to compile Mash on Ubuntu 20.04 LTS I had to make the following changes:

In configure:

Line 2009 - Change the path because apt installs capnp to /usr/bin/ and not /usr/local/bin/: with_capnp=/usr/

In Makefile:

Line 40 - These are the correct paths of these files on x86_64 Ubuntu: /usr/lib/x86_64-linux-gnu/libcapnp.a /usr/lib/x86_64-linux-gnu/libkj.a

With these changes it compiled without errors. I didn't try it yet, but I think it should work..