This is created on analogy with the other CACHE PATHs in this package,
and other `*_INSTALL_INCLUDE_DIR` in other packages.
The branching is adjusted to deduplicate some existing code, and
likewise avoid having to define this new variable more than once.
This will be used for D99484.
Reviewed By: #libunwind, compnerd
Differential Revision: https://reviews.llvm.org/D116873
This adds a CMake option (defaults to OFF to not be intrusive) to activate
2 new targets `install-unwind-headers` and `install-unwind-headers-stripped`.
So, for example:
cmake -S runtimes -B build -G Ninja \
-DLLVM_ENABLE_RUNTIMES='libunwind' \
-DLIBUNWIND_INSTALL_HEADERS=ON
And then, `ninja -C build install-unwind` would install headers in addition
to good ol' dylibs and archives, i.e., targets `install-unwind*` `DEPENDS`
on `install-unwind-headers*`. On the other hand,
`ninja -C build install-unwind-headers` gives you headers only.
Differential Revision: https://reviews.llvm.org/D115535
John Ericson