[libcxxabi][CMake] Add install path variable to allow overriding the destination

[phosek]

Summary

This is going to be used by the runtime build in the multi-target setup to allow using different install prefix for each target.

[beanz]

LGTM

CMakeLists.txt

 set(LIBCXXABI_SOURCE_DIR  ${CMAKE_CURRENT_SOURCE_DIR})
 set(LIBCXXABI_BINARY_DIR  ${CMAKE_CURRENT_BINARY_DIR})
 if (LLVM_LIBRARY_OUTPUT_INTDIR)
   set(LIBCXXABI_LIBRARY_DIR ${LLVM_LIBRARY_OUTPUT_INTDIR})
 else()
   set(LIBCXXABI_LIBRARY_DIR ${CMAKE_BINARY_DIR}/lib${LIBCXXABI_LIBDIR_SUFFIX})
 endif()
 
+set(LIBCXXABI_INSTALL_PREFIX "" CACHE STRING
+    "Define libc++abi destination prefix.")
+
 set(CMAKE_ARCHIVE_OUTPUT_DIRECTORY ${LIBCXXABI_LIBRARY_DIR})
 set(CMAKE_LIBRARY_OUTPUT_DIRECTORY ${LIBCXXABI_LIBRARY_DIR})
 set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${LIBCXXABI_LIBRARY_DIR})
 
 # By default, for non-standalone builds, libcxx and libcxxabi share a library
 # directory.
 if (NOT LIBCXXABI_LIBCXX_LIBRARY_PATH)
   set(LIBCXXABI_LIBCXX_LIBRARY_PATH "${LIBCXXABI_LIBRARY_DIR}" CACHE PATH

src/CMakeLists.txt

set_target_properties(cxxabi_static
                             ON)
   list(APPEND LIBCXXABI_TARGETS "cxxabi_static")
 endif()
 
 # Add a meta-target for both libraries.
 add_custom_target(cxxabi DEPENDS ${LIBCXXABI_TARGETS})
 
 install(TARGETS ${LIBCXXABI_TARGETS}
-  LIBRARY DESTINATION lib${LIBCXXABI_LIBDIR_SUFFIX} COMPONENT cxxabi
-  ARCHIVE DESTINATION lib${LIBCXXABI_LIBDIR_SUFFIX} COMPONENT cxxabi
+  LIBRARY DESTINATION ${LIBCXXABI_INSTALL_PREFIX}lib${LIBCXXABI_LIBDIR_SUFFIX} COMPONENT cxxabi
+  ARCHIVE DESTINATION ${LIBCXXABI_INSTALL_PREFIX}lib${LIBCXXABI_LIBDIR_SUFFIX} COMPONENT cxxabi
   )
 
 if (NOT CMAKE_CONFIGURATION_TYPES)
   add_custom_target(install-cxxabi
     DEPENDS cxxabi
     COMMAND "${CMAKE_COMMAND}"
             -DCMAKE_INSTALL_COMPONENT=cxxabi
             -P "${LIBCXXABI_BINARY_DIR}/cmake_install.cmake")
 
   # TODO: This is a legacy target name and should be removed at some point.
   add_custom_target(install-libcxxabi DEPENDS install-cxxabi)
 endif()