The relative path check added by commit 5838aba1 (Export: Report error
on relative include with genex., 2013-11-26, v3.0.0-rc1~285^2) was added
one condition too early.  If the value starts in `${_IMPORT_PREFIX}`
(which comes from `$<INSTALL_PREFIX>`) then it is an absolute path.

Fixes: #19791