Mir doesn't install cleanly if docs are not built

Build Mir with a cmake configuration having -DBUILD_DOXYGEN=NO, like this one (which I used):

cmake ${mir_code_dir} -DCMAKE_INSTALL_PREFIX=/usr -DBUILD_DOXYGEN=NO -DCMAKE_INSTALL_LIBEXECDIR="lib/$(dpkg-architecture -qDEB_HOST_MULTIARCH)/mir" -DMIR_PLATFORM=android -DMIR_ENABLE_TESTS=NO

then make.

When you do "make install", you get an error like this:

"""
Install the project...
-- Install configuration: ""
CMake Error at cmake_install.cmake:44 (FILE):
  file INSTALL cannot find
  "/home/phablet/mir/for-qpamirserverNew_build/doc/html".
"""

To work around it, you have to mkdir the missing "doc/html" dir and "make install" again.