[CMake] Set CMake policy CMP0051 to OLD

[zturner]

Summary

When you use generator expressions in a library sources list,
and then later access the SOURCES property, the OLD behavior
(CMake 3.0 and earlier) would not include these expressions in
the SOURCES property. The NEW behavior (starting in CMake 3.1)
is that they do include the generator expressions in the SOURCES
property.

You can see an example of this warning occurring in practice on the
lldb bots currently.

http://lab.llvm.org:8011/builders/lldb-x86-windows-msvc/builds/2382/steps/cmake/logs/stdio

[beanz]

LGTM.

As may have been noted by my comments on LLVMDev today. I really think we need to come up with a plan for updating CMake more frequently. Especially if we drop autoconf.

CMakeLists.txt

   set(CMAKE_BUILD_TYPE "Debug")
 endif()
 
+if (POLICY CMP0051)
+  # CMake 3.1 and higher include generator expressions of the form
+  # $<TARGETLIB:obj> in the SOURCES property.  These need to be
+  # stripped everywhere that access the SOURCES property, so we just
+  # defer to the OLD behavior of not including generator expressions
+  # in the output for now.
+  cmake_policy(SET CMP0051 OLD)
+endif()
+
 if(POLICY CMP0022)
   cmake_policy(SET CMP0022 NEW) # automatic when 2.8.12 is required
 endif()