[CMake] Update to requiring CMake 3.4.3

Summary:
This is as per the discussions on developer lists:

http://lists.llvm.org/pipermail/llvm-dev/2016-April/098780.html
http://lists.llvm.org/pipermail/llvm-dev/2016-May/100058.html

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D20828

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@271329 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed