In commit 9fc20a4f3e (install: Add sane set of defaults for DESTINATION
and file type parameters, 2018-11-02, v3.14.0-rc1~410^2~1), a regression
was introduced, in which an `install(TARGETS)` with a
RUNTIME/LIBRARY/ARCHIVE DESTINATION but no PUBLIC_HEADER/PRIVATE_HEADER
DESTINATION would then install the headers. The old behavior did not do
this. Restore the old behavior.
Fixes: #20326
If the user does not specify a DESTINATION for a target type, the
install() command checks to see if the appropriate variable from
GNUInstallDirs is set. If it is not, then it uses an appropriate
hard-coded guess.
In addition, for FILES and DIRECTORY, the user can specify a file
type instead of a DESTINATION, and the command will use the
appropriate variable from GNUInstallDirs, or a hard-coded guess if
it is not set.
