GNU-FindBinUtils: Avoid setting policy CMP0054

The `cmake_policy(SET)` call triggers `CMP0011` warnings when this
module is used in a project that does not set `CMP0011` to NEW.  We
could avoid these with a `cmake_policy` PUSH/POP pair, but it is simpler
to adjust our code to not trigger `CMP0054` in the first place.

Modules/Compiler/GNU-FindBinUtils.cmake

-cmake_policy(SET CMP0054 NEW)
-
-if("${_CMAKE_PROCESSING_LANGUAGE}" STREQUAL "")
+if(NOT DEFINED _CMAKE_PROCESSING_LANGUAGE OR _CMAKE_PROCESSING_LANGUAGE STREQUAL "")
   message(FATAL_ERROR "Internal error: _CMAKE_PROCESSING_LANGUAGE is not set")
 endif()