[CMake] xcode: rebuild project when dirty

[Tim Blechmann]

hi all,

the make/ninja files and visual studio project seem to be rebuilt
automatically when some CMakeLists.txt files change. this does not seem
to be the case for xcode projects, though. to regenerate the project, i
need to buid the ZERO_CHECK target explicitly.

is this a bug or a feature?

thnx,
tim