Add support for precompiled headers

[GPMueller]

The ability to precompile headers was recently added to CMake 3.16 (docs), and we should add support in some fashion, too.

Clang documentation is here and here.

[GPMueller]

I would suggest the following approach, close to what CMake does:

• for each target for which we wish to precompile headers, we gather the header files into one big build/.../targetname/buildtype/pch/combination.h|hpp
• the combination-header is compiled as clang++ -x c++-header combination.hpp -o combination.hpp.pch + flags (note we need to distinguish between languages)
• the pch is included to the sources using the flag -include combination.hpp
• for header-only targets, the -include flag should go into the interface_include_directories?

Notes:

• it seems that, unfortunately, we cannot compile multiple headers into a single pch
• clang will not automatically include a pch instead of a regular include, meaning the -include flag is necessary for all sources which should use the pch. This makes it a little more difficult to use correctly and consistently

Questions:

• should we always precompile headeronly libraries?
• should we always precompile any headers?
• should we add some kind of global or per-target switch to activate precompilation of headers?
• should precompiled headers of dependencies always be used by dependent targets?
• I suppose we do not want to allow installing pch's, but is there any other case in which we do not want to use pch's?