Build is broken

[mihaibudiu]

p4c has tweaked the cmakefiles recently and now this build is broken:

-- Start configuring OFP4 back end
CMake Error at extensions/ofp4/CMakeLists.txt:51 (build_unified):
  Unknown CMake command "build_unified".

[blp]

This is fundamentally going to keep happening (whether for build system or otherwise) as p4c advances. Do you think it would be reasonable to require a particular version or commit of p4c?

[mihaibudiu]

We don't really touch the build system much, but some overly zealous people did. I think we have a few years of stability from now on