macOS: Reject setting FRAMEWORK after install()
FRAMEWORK target property affects the way the
treats the target and so should be set first. Our implementation
assumed that this was always the case led to an assertion failure.
Fully supporting out-of-order specification will require non-trivial
refactoring to defer install generator creation to generate time.
For now simply reject the case with an error instead of failing an
Fixes: #18848 (closed)