[LyX/master] Cmake build: Disable possible warn about not known policy

[Kornel Benko]

commit 6343452a7397a5ac4b84af30d61c4d7fca5afbc1
Author: Kornel Benko <kor...@lyx.org>
Date:   Sun Jun 10 11:40:12 2018 +0200

    Cmake build: Disable possible warn about not known policy
---
 CMakeLists.txt |    4 +++-
 1 files changed, 3 insertions(+), 1 deletions(-)

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 410f0af..d1b448c 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -48,7 +48,9 @@ if(COMMAND cmake_policy)
                cmake_policy(SET CMP0043 NEW)
        endif()
        cmake_policy(SET CMP0020 NEW)
-        cmake_policy(SET CMP0075 OLD)
+       if(POLICY CMP0075)
+               cmake_policy(SET CMP0075 OLD)
+       endif()
 endif()
 
 set(CMAKE_ALLOW_LOOSE_LOOP_CONSTRUCTS true)