Fix CMake CMP0135 warnings for ExternalProject downloads (#5083)

Set cmake_policy(SET CMP0135 NEW) so that ExternalProject_Add URL
downloads use extraction-time timestamps instead of archive timestamps.
This silences the dev warnings from gbenchmark and googletest downloads.

Fixes #4852

Author: mvanhorn

lib/CMakeLists.txt

@@ -5,6 +5,10 @@ project(ponyclibs VERSION ${PONYC_PROJECT_VERSION} LANGUAGES C CXX)
 
 include(ExternalProject)
 
+if(POLICY CMP0135)
+    cmake_policy(SET CMP0135 NEW)
+endif()
+
 if(NOT DEFINED PONYC_LIBS_BUILD_TYPE)
     set(PONYC_LIBS_BUILD_TYPE Release)
 endif()