[libunwind] Switch to add_llvm_install_targets

[smeenai]

Summary

This gains us the install-unwind-stripped target, to perform stripping
during installation.

[beanz]

LGTM.

[dtzWill]

Hmm, this change means very recent LLVM is required to build libunwind-- previously I was able to use recent libunwind/libcxx/libcxxabi with LLVM 5 and LLVM 4.

Other runtime projects (compiler-rt, libcxx, libcxxabi) were modified to support the new "strip-and-install" targets such that they didn't depend on LLVM cmake bits, could that sort of approach work here?

[smeenai]

The other runtime projects had the stripped installation targets added manually because they officially support being built completely standalone (i.e. without any LLVM components), so we can't rely on LLVM's CMake there. That's really unfortunate IMO, since it leads to a bunch of duplication, but it is what it is.

libunwind doesn't support building completely standalone AFAIK, so I think assuming trunk libunwind will be paired with trunk LLVM is pretty reasonable in general. It's easy enough to not break your use case here, however, so I went ahead and did so in r320163.

[dtzWill]

Okay, that makes sense. Thanks for accommodating my use-case anyway! :D.

src/CMakeLists.txt

 
 if (LIBUNWIND_INSTALL_LIBRARY)
   install(TARGETS ${LIBUNWIND_TARGETS}
     LIBRARY DESTINATION ${LIBUNWIND_INSTALL_PREFIX}lib${LIBUNWIND_LIBDIR_SUFFIX} COMPONENT unwind
     ARCHIVE DESTINATION ${LIBUNWIND_INSTALL_PREFIX}lib${LIBUNWIND_LIBDIR_SUFFIX} COMPONENT unwind)
 endif()
 
 if (NOT CMAKE_CONFIGURATION_TYPES AND LIBUNWIND_INSTALL_LIBRARY)
-  add_custom_target(install-unwind
+  add_llvm_install_targets(install-unwind
                            DEPENDS unwind
-    COMMAND "${CMAKE_COMMAND}"
-            -DCMAKE_INSTALL_COMPONENT=unwind
-            -P "${LIBUNWIND_BINARY_DIR}/cmake_install.cmake")
+                           COMPONENT unwind)
 endif()