Avoid error when using CMake pre-3.0.

Check for policy CMP0042.  If it is not known, don't try to set it.
This check-before-set idiom is suggested by the CMake documentation.
See <https://cmake.org/cmake/help/v3.0/manual/cmake-policies.7.html>.

Without this check, CMake pre-3.0 gives an error.

BTW, did you know that CMake 2.8 is the default on CentOS 7.3?

runtime/CMakeLists.txt

@@ -32,7 +32,10 @@
 cmake_minimum_required(VERSION 2.8.12)
 project(csmith_runtime)
 
-cmake_policy(SET CMP0042 NEW) # Quiet warning about shared libs on OS X.
+# CMP0042 was introduced in CMake 3.0.  Don't error in older CMakes.
+if(POLICY CMP0042)
+  cmake_policy(SET CMP0042 NEW) # Quiet warning about shared libs on OS X.
+endif()
 
 set(INCLUDE_DIR "${CMAKE_INSTALL_PREFIX}/include/csmith-${csmith_VERSION}")
 set(LIB_DIR     "${CMAKE_INSTALL_PREFIX}/lib")