QNX: Fix autogen compiler predefines detection

In commit v3.12.0-rc1~20^2 (Move GNU COMPILER_PREDEFINES_COMMAND from
Platform to Compiler, 2018-06-01), `Compiler/GNU.cmake` introduced the
`-dM` flag, which requires `-Wp` prefix for the QNX compiler wrapper.

Modules/Compiler/QCC.cmake

@@ -16,6 +16,8 @@ macro(__compiler_qcc lang)
   set(_CMAKE_${lang}_IPO_SUPPORTED_BY_CMAKE NO)
   set(_CMAKE_${lang}_IPO_MAY_BE_SUPPORTED_BY_COMPILER NO)
 
+  set(CMAKE_${lang}_COMPILER_PREDEFINES_COMMAND "${CMAKE_${lang}_COMPILER}" "-Wp,-dM" "-E" "-c" "${CMAKE_ROOT}/Modules/CMakeCXXCompilerABI.cpp")
+
   unset(CMAKE_${lang}_COMPILE_OPTIONS_IPO)
   unset(CMAKE_${lang}_ARCHIVE_CREATE_IPO)
   unset(CMAKE_${lang}_ARCHIVE_APPEND_IPO)