[cmake] Define WINPR_DEFINE_ATTR_NODISCARD

* Do not define WINPR_ATTR_NODISCARD if WINPR_DEFINE_ATTR_NODISCARD is
  not defined.
* Define WINPR_DEFINE_ATTR_NODISCARD via CMake to enable this for
  internal use only

CMakeLists.txt

@@ -96,6 +96,9 @@ include(SetFreeRDPCMakeInstallDir)
 include(Doxygen)
 include(GetSysconfDir)
 
+# FreeRDP internal builds should always check results
+add_compile_definitions(WINPR_DEFINE_ATTR_NODISCARD)
+
 # Soname versioning
 set(BUILD_NUMBER 0)
 if($ENV{BUILD_NUMBER})