Inconsistent install directory

[pasbi]

Following script:

cmake \
  -GNinja \
  -S $src  \
  -B $build \
  -DCMAKE_INSTALL_PREFIX=/tmp/bar

cmake --build $build
cmake --install $build --prefix /tmp/foo

produces following:

-- Install configuration: ""
-- Up-to-date: /tmp/foo/lib/libQGLViewer.so
-- Up-to-date: /tmp/bar/include/QGLViewer/camera.h
-- Up-to-date: /tmp/bar/include/QGLViewer/config.h
-- Up-to-date: /tmp/bar/include/QGLViewer/constraint.h
...

I.e., the library is written into the correct path supplied with --prefix in the install directive, while the headers are installed into the -DCMAKE_INSTALL_PREFIX.

The CMake documentation clearly says that --prefix in an --install directive overwrites -DCMAKE_PREFIX_PATH.

The problem is, as far as I can tell, the usage of CMAKE_INSTALL_FULL_INCLUDEDIR, which is expanded at configuration time to an absolute path.
So there's no way for --prefix to change that in a later change.
CMAKE_INSTALL_LIBDIR, however, expands only to a relative path (lib) which is properly handled by the install script later.

GNUInstallDirs documentation also warns about this issue:

These variables shouldn't be used in install() commands as they do not work with the cmake --install command's --prefix option, or with the cpack installer generators.