Comment 6 for bug 1523739

https://github.com/PintaProject/Pinta/commit/2e33aade looks good. No more "[...] `Unix' does not exist in the namespace `Mono' [...] errors.

Unfortunately this commit gives me a new error when building:

: warning : Default tasks file /usr/lib/mono/3.5/Microsoft.Common.tasks not found, ignoring.
[...]
: error : Error initializing task MakeDir: Not registered task MakeDir.

Workaround:
sudo mkdir /usr/lib/mono/3.5/
sudo ln -s /usr/lib/mono/4.5/Microsoft.Common.tasks /usr/lib/mono/3.5/

But this workaround isn't needed for building the preceding commit 7c34b07 when using the Pinta-1.6_mono4-patch-script.sh script I provided. (just tested again)
https://bugs.launchpad.net/pinta/+bug/1523739/comments/1