[QGIS-Developer] Build fails on Ubuntu 18.04

[Clemens Raffler]

Dear Devs,

I am currently working on some processing algorithms and wanted to 
rebase my branch on master. But since yesterdays pull of the latest 
commits (some also concerning a fix of the windows build), cmake 
complains that 'Protobuf library's 'protoc' tool was not found!'. Does 
anyone know the cause for this problem or how to fix it? I am working on 
Ubuntu 18.04.4.

Thanks!

Best regards,
Clemens