This matches the decision made in D99697.
It also shouldn't reintroduce the issue fixed in D99636.
The variable was originally introduced in
b22f448c21e718a3b6219df89169f38d436189c6 but is not essential to that
change.
Once we finish adding GnuInstallDirs support in D100810 and D99484,
setting CMAKE_INSTALL_LIBDIR would also work to change the
installation directory (though for more than libc).
GnuInstallDirs support also brings up an issue which is avoided if
variables like LIBC_INSTALL_PREFIX don't exist. Because the
GnuInstallDirs variables can be absolute paths, it is a bit unclear
how the per-project prefixes would work: does the project-agnostic
role-specific variable (e.g. CMAKE_INSTALL_LIBDIR), or project-specfic
role-agnostic (e.g. LIBC_INSTALL_PREFIX) take priority? Each is more
specific than the other on one axis, but not the other.