[LyX/master] CMake build: disable warn about not known policy

[Scott Kostyshak]

commit d35753a6d0e1bd0287c88998ffc9cb874279536b
Author: Scott Kostyshak <skostysh at lyx.org>
Date:   Thu Nov 10 10:34:15 2022 -0500

    CMake build: disable warn about not known policy
    
    This only showed up with -DLYX_EXTERNAL_ICONV=OFF.
---
 3rdparty/libiconv/CMakeLists.txt |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)

diff --git a/3rdparty/libiconv/CMakeLists.txt b/3rdparty/libiconv/CMakeLists.txt
index 39beb96..12d5446 100644
--- a/3rdparty/libiconv/CMakeLists.txt
+++ b/3rdparty/libiconv/CMakeLists.txt
@@ -14,6 +14,9 @@ if (POLICY CMP0069)
     set(LYX_IPO_SUPPORTED YES)
   endif()
 endif()
+if (POLICY CMP0075)
+  cmake_policy(SET CMP0075 NEW)
+endif()
 
 set(CMAKE_MODULE_PATH .)
 include ( configure.cmake )