Fix cmake for zlib

[wenlei]

Summary

system_libs should be set to library names instead of library path. find_package(ZLIB) populates ZLIB_LIBRARY with path and that evetually gets rolled into INTERFACE_LINK_LIBRARIES of LLVMSupport which made it not portable.

[mgorny]

Is there a way to get the basename from ZLIB_LIBRARY by any chance? Not saying it's actually necessary, just wondering.

[wenlei]

In D72490#1813569, @mgorny wrote:

Is there a way to get the basename from ZLIB_LIBRARY by any chance? Not saying it's actually necessary, just wondering.

That would be trying to recover z from a/b/c/libz.so. Given how system_libs is set for other libs (line 8), I think it's better to just set it directly.

I should also mention that the problem was introduced in this change: https://github.com/llvm/llvm-project/commit/abb00753069554c538f3d850897373d093389945#diff-cb177ae2458d3cc8a8e499510ab787c2

[compnerd]

I'm not sure I understand the motivation for this change. Can you please expand on that? How are you using it that it doesn't work for you? Additionally, z is definitely the wrong name, please compute that by using get_file_name_component, CMAKE_SHARED_LIBRARY_PREFIX, and string(REGEX...)

[wenlei]

I'm not sure I understand the motivation for this change. Can you please expand on that? How are you using it that it doesn't work for you?

@compnerd, the first problem is system_libs will be propagated into INTERFACE_LINK_LIBRARIES which is supposed to be relocatable (https://cmake.org/cmake/help/v3.3/prop_tgt/INTERFACE_LINK_LIBRARIES.html). However ZLIB_LIBRARY contains the absolute path zlib, so having ZLIB_LIBRARY in system_libs is wrong as it leads to non-relocatable interface libs.

Additionally, z is definitely the wrong name, please compute that by using get_file_name_component, CMAKE_SHARED_LIBRARY_PREFIX, and string(REGEX...)

The problem also goes beyond absolute path vs relative path. If you look at line 11 for example, the way these libraries are described is like this: rt for librt.so. So we had to revert it back to use z for zlib.so for our usage of LLVM libs. What you suggested sounds like a good idea, but I won't be able to revisit this any time soon. So feel free to take over the patch. Thanks.

llvm/lib/Support/CMakeLists.txt

 if(LLVM_ENABLE_ZLIB)
-  set(system_libs ${ZLIB_LIBRARY})
+  set(system_libs z)
 endif()
 
 if( MSVC OR MINGW )
   # libuuid required for FOLDERID_Profile usage in lib/Support/Windows/Path.inc.
   # advapi32 required for CryptAcquireContextW in lib/Support/Windows/Path.inc.
   set(system_libs ${system_libs} psapi shell32 ole32 uuid advapi32)
 elseif( CMAKE_HOST_UNIX )
   if( HAVE_LIBRT )