Where should platform-dependent header files be installed?

Question

For example, if a header defines something based on configure checks which depend on the platform (e.g. different for x86 and x86-64), where should it be installed?

Answer 1

You don't install header files.

You can place them wherever you want. In your install script, your conditional will just point to the appropriate files when taking the correct branch. You can put any file path you want in these conditionals.