install_headers() should also install headers into intermediate directories

[CoffeeFlux]

Copying from #2546, which was closed but AFAIK without the problem being resolved.

install_headers() and configure_file() with the install_dir option set should install headers into meson-private/include-intermediates during the meson config phase .

Many 3rd packages have header files and source files in same directory but install header files into a subdirectory after install, so this will cause an error finding the header files when using them in a subproject.

The workaround, as suggested by @nirbheek:

Yes, there is a way around this. You can copy the headers to the build directory into the subdir that you use when installed with configure_file(), and then create an include_directories() object that points to it.

[eli-schwartz]

Duplicate of #2546