Check the complete include path for being a system include, not
the derived framework search path. The code for Ninja and Makefile
generators does exactly the same.
Fixes: #23011
The `FRAMEWORK` target property affects the way the `install()` command
treats the target and so should be set first. Our implementation
assumed that this was always the case and led to an assertion failure.
Prior to CMake 3.12 this was visible only when using an explicit
`LIBRARY ... NAMELINK_ONLY` option, but commit 0212d7c762 (install: add
NAMELINK_COMPONENT argument, 2018-04-18, v3.12.0-rc1~139^2~3) made
it possible with a simple `LIBRARY DESTINATION`.
Fully supporting out-of-order specification will require non-trivial
refactoring to defer install generator creation to generate time.
For now simply restore the old behavior of installing the framework
to the library destination and warn about the case.
Fixes: #18848
