WCDH: Restore definition for <prefix>_COMPILER_IS_MIPSpro

Code removed for MIPSpro by commit 214fcefa52 (Remove now-unused code
once used for MIPSpro on IRIX, 2019-02-21) actually changed a
public-facing API by dropping the `<prefix>_COMPILER_IS_MIPSpro`
definition from the generated compiler detection header.  Restore the
definition hard-coded to `0` since the compiler will never be MIPSpro.

Reported-by: Hans Johnson <[email protected]>

Modules/CMakeCompilerIdDetection.cmake

@@ -96,6 +96,8 @@ function(compiler_id_detection outvar lang)
       foreach(Id ${ordered_compilers})
         string(APPEND CMAKE_${lang}_COMPILER_ID_CONTENT "# define ${CID_PREFIX}COMPILER_IS_${Id} 0\n")
       endforeach()
+      # Hard-code definitions for compilers that are no longer supported.
+      string(APPEND CMAKE_${lang}_COMPILER_ID_CONTENT "# define ${CID_PREFIX}COMPILER_IS_MIPSpro 0\n")
     endif()
 
     set(pp_if "#if")