Add env var for flatc schema compiler cmake module

CMake/BuildFlatBuffers.cmake

@@ -63,6 +63,13 @@ function(build_flatbuffers flatbuffers_schemas
     set(FLATC_TARGET flatc)
     set(FLATC flatc)
   endif()
+  set(FLATC_SCHEMA_ARGS --gen-mutable)
+  if(FLATBUFFERS_FLATC_SCHEMA_EXTRA_ARGS)
+    set(FLATC_SCHEMA_ARGS
+      ${FLATBUFFERS_FLATC_SCHEMA_EXTRA_ARGS}
+      ${FLATC_SCHEMA_ARGS}
+      )
+  endif()
 
   set(schema_glob "*.fbs")
   # Generate the include files parameters.
@@ -86,7 +93,7 @@ function(build_flatbuffers flatbuffers_schemas
       set(generated_include ${generated_includes_dir}/${filename}_generated.h)
       add_custom_command(
         OUTPUT ${generated_include}
-        COMMAND ${FLATC} --gen-mutable
+        COMMAND ${FLATC} ${FLATC_SCHEMA_ARGS}
         -o ${generated_includes_dir}
         ${include_params}
         -c ${schema}