Suggestions for the Wiki

I suggest to add the following pieces to https://github.com/rr-debugger/rr/wiki/Building-And-Installing:

• cmake define disable32bit, reasoning: builds faster, has less dependencies, 64bit recording and replay is quite likely happening on most systems, works around #2936 (it possibly also could default to ON instead of OFF and then be documented there as an extra sub-header "build to record and replay 32bit applications"...)
• cmake define CMAKE_INSTALL_PREFIX; while this one is likely known to people that are familiar with cmake, it still would be possibly useful to know for others;
  I've personally tried make install PREFIX=/myprefix first, then CMAKE_INSTALL_PREFIX=/myprefix cmake, but "of course" the correct one is cmake -DCMAKE_INSTALL_PREFIX=/myprefix (while the correct way to specify the C compiler is CC=gcc cmake)

Open the rr Wiki's Building-And-Installing page and review the requested CMake options: disable32bit and CMAKE_INSTALL_PREFIX. Document their purpose and the relevant command-line forms, including the 32-bit build tradeoff; the page should clearly explain how readers can use both options.