Merge changes I2de7d14d,I56392340 into ub-games-master

* changes:
  Added accessor for file_extension in generated code.
  Added missing --gen-mutable to CMakeLists.txt
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 32287ea..39e65fa 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -95,7 +95,7 @@
   add_executable(flatc ${FlatBuffers_Compiler_SRCS})
 endif()
 
-if(FLATBUFFERS_BUILD_FLATC)
+if(FLATBUFFERS_BUILD_FLATHASH)
   add_executable(flathash ${FlatHash_SRCS})
 endif()