[LyX/master] Cmake build: Remove warnings about deprecated policy CMP0075

[Kornel Benko]

commit 348a3be5402bd4290a7ed14a185713c50a526790
Author: Kornel Benko <kornel at lyx.org>
Date:   Sat Mar 7 13:49:35 2020 +0100

    Cmake build: Remove warnings about deprecated policy CMP0075
---
 3rdparty/zlib/CMakeLists.txt |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)

diff --git a/3rdparty/zlib/CMakeLists.txt b/3rdparty/zlib/CMakeLists.txt
index 9bc1d0d..e81e43a 100644
--- a/3rdparty/zlib/CMakeLists.txt
+++ b/3rdparty/zlib/CMakeLists.txt
@@ -7,6 +7,9 @@ if (POLICY CMP0069)
     set(LYX_IPO_SUPPORTED YES)
   endif()
 endif()
+if (POLICY CMP0075)
+  cmake_policy(SET CMP0075 NEW)
+endif()
 
 set(CMAKE_ALLOW_LOOSE_LOOP_CONSTRUCTS ON)