install: Normalize DESTINATION paths

The file generated by install(EXPORT) computes _IMPORT_PREFIX
in a way that assumes a normalized path. If the DESTINATION
contains any ../ components, the computed _IMPORT_PREFIX
would be wrong. Force the DESTINATION path to be normalized,
subject to the new CMP0176 policy.

Also normalize all other DESTINATION paths for consistency,
except for INCLUDES DESTINATION, which is not strictly a
destination but rather a search path to add.

Fixes: #26252

Help/command/install.rst

@@ -58,7 +58,7 @@ signatures that specify them.  The common options are:
   ``<dir>`` should be a relative path.  An absolute path is allowed,
   but not recommended.
 
-  When a relative path is given it is interpreted relative to the value
+  When a relative path is given, it is interpreted relative to the value
   of the :variable:`CMAKE_INSTALL_PREFIX` variable.
   The prefix can be relocated at install time using the ``DESTDIR``
   mechanism explained in the :variable:`CMAKE_INSTALL_PREFIX` variable
@@ -75,6 +75,11 @@ signatures that specify them.  The common options are:
   If an absolute path (with a leading slash or drive letter) is given
   it is used verbatim.
 
+  .. versionchanged:: 3.31
+    ``<dir>`` will be normalized according to the same
+    :ref:`normalization rules <Normalization>` as the
+    :command:`cmake_path` command.
+
 ``PERMISSIONS <permission>...``
   Specify permissions for installed files.  Valid permissions are
   ``OWNER_READ``, ``OWNER_WRITE``, ``OWNER_EXECUTE``, ``GROUP_READ``,
@@ -396,6 +401,12 @@ Signatures
     If a relative path is specified, it is treated as relative to the
     :genex:`$<INSTALL_PREFIX>`.
 
+    Unlike other ``DESTINATION`` arguments for the various ``install()``
+    subcommands, paths given after ``INCLUDES DESTINATION`` are used as
+    given.  They are not normalized, nor assumed to be normalized, although
+    it is recommended that they are given in normalized form (see
+    :ref:`Normalization`).
+
   ``RUNTIME_DEPENDENCY_SET <set-name>``
     .. versionadded:: 3.21
 
@@ -807,6 +818,7 @@ Signatures
   the generated file will be called ``<export-name>.cmake`` but the ``FILE``
   option may be used to specify a different name.  The value given to
   the ``FILE`` option must be a file name with the ``.cmake`` extension.
+
   If a ``CONFIGURATIONS`` option is given then the file will only be installed
   when one of the named configurations is installed.  Additionally, the
   generated import file will reference only the matching target