[cmake] [libcxx] Call llvm_setup_rpath() when adding shared libraries.

[hintonda]

Summary

Clang and llvm already use llvm_setup_rpath(), so this change will
help standarize rpath usage across all projects.

[hintonda]

Reopen after rollback.

@smeenai pointed out LLVM may not be available in standalone builds

[hintonda]

I probably should have put this in the summary, but the reason for this patch isn't just to clean up the cmake files, it's to fix a cmake configuration failure when cross-compiling for Linux on Darwin:

CMake Error at /Users/dhinton/projects/llvm_project/libcxx/lib/CMakeLists.txt:224 (add_library):

The install of the cxx_shared target requires changing an RPATH from the
build tree, but this is not supported with the Ninja generator unless on an
ELF-based platform.  The CMAKE_BUILD_WITH_INSTALL_RPATH variable may be set
to avoid this relinking step.

[hintonda]

Only call llvm_setup_rpath() if LLVM_FOUND is true.

[ldionne]

@hintonda Is there a reason why this was done except for "standardizing the usage across projects"?

I'm asking because this change prevents us from being able to avoid having a rpath completely when building libc++. That is the behavior we want when building libc++ as a system library, in which case we specify the install name dir to be /usr/lib, and we don't want any rpath to appear.

[hintonda]

In D42459#2383753, @ldionne wrote:

@hintonda Is there a reason why this was done except for "standardizing the usage across projects"?

It was just for standardization. Please feel free to rollback or modify as necessary.

[ldionne]

In D42459#2383794, @hintonda wrote:

In D42459#2383753, @ldionne wrote:

@hintonda Is there a reason why this was done except for "standardizing the usage across projects"?

It was just for standardization. Please feel free to rollback or modify as necessary.

Ok, thanks a lot for the information. If things worked before, I would like to revert this change as we are working around it in our internal build, and it just bit me today. I'll send a review.

[ldionne]

I proposed a revert here for those who are interested: https://reviews.llvm.org/D91099

lib/CMakeLists.txt

PROPERTIES
     COMPILE_FLAGS "${LIBCXX_COMPILE_FLAGS}"
 )
 
 set(LIBCXX_TARGETS)
 
 # Build the shared library.
 if (LIBCXX_ENABLE_SHARED)
   add_library(cxx_shared SHARED $<TARGET_OBJECTS:cxx_objects>)
+  llvm_setup_rpath(cxx_shared)
   target_link_libraries(cxx_shared ${LIBCXX_LIBRARIES})
   set_target_properties(cxx_shared
     PROPERTIES
       LINK_FLAGS    "${LIBCXX_LINK_FLAGS}"
       OUTPUT_NAME   "c++"
       VERSION       "${LIBCXX_ABI_VERSION}.0"
       SOVERSION     "${LIBCXX_ABI_VERSION}"
   )