miscellaneous changes [4-2-stable]

[jassigill2000]

• support added to build the plugin using custom externals from a directory/repo
• cleaned up the cmake file
• cleaned up the test hook
• fixed the failing test by adding the right amount of escapes

[trel]

seems good to me - please # all four.

[jassigill2000]

tests were ran on ubuntu 16, ubuntu 18 and centos 7 the changes are mergeable.

[trel]

please do the same for master