tools: Make phantomjs-prebuilt an optional dependency
tools: Make phantomjs-prebuilt an optional dependency

Tests won't run if it fails, but we shouldn't require it for
building.

Closes #6430
Fixes #6428
SotolitoLabs/cockpit,Scribery/cockpit,cockpituous/cockpit,martinpitt/cockpit,Scribery/cockpit,martinpitt/cockpit,harishanand95/cockpit,larskarlitski/cockpit,moraleslazaro/cockpit,SotolitoLabs/cockpit,harishanand95/cockpit,andreasn/cockpit,michalskrivanek/cockpit,moraleslazaro/cockpit,xhad/cockpit,mareklibra/cockpit,larsu/cockpit,garrett/cockpit,xhad/cockpit,harishanand95/cockpit,Scribery/cockpit,SotolitoLabs/cockpit,deryni/cockpit,garrett/cockpit,moraleslazaro/cockpit,garrett/cockpit,dperpeet/cockpit,dperpeet/cockpit,michalskrivanek/cockpit,dperpeet/cockpit,andreasn/cockpit,Scribery/cockpit,cockpit-project/cockpit,andreasn/cockpit,mareklibra/cockpit,larskarlitski/cockpit,Scribery/cockpit,cockpituous/cockpit,harishanand95/cockpit,andreasn/cockpit,larsu/cockpit,cockpituous/cockpit,martinpitt/cockpit,Scribery/cockpit,larsu/cockpit,cockpit-project/cockpit,moraleslazaro/cockpit,larsu/cockpit,moraleslazaro/cockpit,deryni/cockpit,andreasn/cockpit,dperpeet/cockpit,petervo/cockpit,cockpituous/cockpit,mareklibra/cockpit,mvollmer/cockpit,stefwalter/cockpit,petervo/cockpit,mvollmer/cockpit,larskarlitski/cockpit,martinpitt/cockpit,moraleslazaro/cockpit,dperpeet/cockpit,SotolitoLabs/cockpit,mareklibra/cockpit,SotolitoLabs/cockpit,michalskrivanek/cockpit,garrett/cockpit,stefwalter/cockpit,SotolitoLabs/cockpit,mvollmer/cockpit,michalskrivanek/cockpit,xhad/cockpit,xhad/cockpit,larskarlitski/cockpit,deryni/cockpit,larsu/cockpit,cockpit-project/cockpit,petervo/cockpit,deryni/cockpit,xhad/cockpit,cockpituous/cockpit,Scribery/cockpit,larskarlitski/cockpit,larskarlitski/cockpit,michalskrivanek/cockpit,SotolitoLabs/cockpit,stefwalter/cockpit,petervo/cockpit,petervo/cockpit,xhad/cockpit,michalskrivanek/cockpit,deryni/cockpit,stefwalter/cockpit,petervo/cockpit,harishanand95/cockpit,andreasn/cockpit,mareklibra/cockpit,larsu/cockpit,mareklibra/cockpit,stefwalter/cockpit,harishanand95/cockpit,michalskrivanek/cockpit,cockpituous/cockpit,petervo/cockpit,mvollmer/cockpit,dperpeet/cockpit,xhad/cockpit,deryni/cockpit,larskarlitski/cockpit,garrett/cockpit,moraleslazaro/cockpit,mvollmer/cockpit,stefwalter/cockpit,deryni/cockpit,dperpeet/cockpit,cockpit-project/cockpit,mareklibra/cockpit,andreasn/cockpit,martinpitt/cockpit,harishanand95/cockpit,stefwalter/cockpit,cockpit-project/cockpit