This simplifies building without a build system.
This is in line with #925; the paths fixed here were either forgotten
there or only introduced later. This commit also makes the Makefile
stricter so that further "wrong" #include paths will lead to build
errors even in autotools builds.
This belongs to #929.
Co-authored-by: Hennadii Stepanov <32963518+hebasto@users.noreply.github.com>
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