Add test.
Merge topic 'fix-genex-preprocess' into next

875586b Add test.
Fix cmGeneratorExpression::Preprocess for interleaved inputs.

We can't find both preprocessing expressions at once, because then
the BUILD_INTERFACE will always be favored if both are present, even
if INSTALL_INTERFACE appears first.

This was affecting the behavior of install(EXPORT) because the
INTERFACE_INCLUDE_DIRECTORIES contained entries like

 /foo/include;$<INSTALL_INTERFACE:/bar/include>

As the INSTALL_INTERFACE always evaluates to '0', it always needs
to be preprocessed properly.
Merge topic 'fix-genex-preprocess' into next

7a619fa Fix cmGeneratorExpression::Preprocess for interleaved inputs.