CMake: Fix policy warning about OSL_ROOT being set.

When the OSL_ROOT variable is set this is ignored
by findpackage on cmake < 3.12. CMake 3.12 and up
also  ignore it and warn about it. This change
tells cmake it is OK to use the variable and
stop warning

CMakeLists.txt

@@ -102,6 +102,11 @@ if(POLICY CMP0068)
   cmake_policy(SET CMP0068 NEW)
 endif()
 
+# find_package() uses <PackageName>_ROOT variables.
+if(POLICY CMP0074)
+  cmake_policy(SET CMP0074 NEW)
+endif()
+
 #-----------------------------------------------------------------------------
 # Load some macros.
 include(build_files/cmake/macros.cmake)