add setloadfile and setfileexists functions to flatbuffers

to allow for custom file loaders. The targeted use case is android runtime.

(from CL 116980408)

Change-Id: I8785c0acf714fab41d8f6fc9f1c52875423b8f5b
diff --git a/CMakeLists.txt b/CMakeLists.txt
index d2edf82..70a7dd5 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -26,6 +26,7 @@
   src/idl_parser.cpp
   src/idl_gen_text.cpp
   src/reflection.cpp
+  src/util.cpp
 )
 
 set(FlatBuffers_Compiler_SRCS
@@ -68,6 +69,7 @@
   include/flatbuffers/util.h
   src/idl_parser.cpp
   src/idl_gen_text.cpp
+  src/util.cpp
   samples/sample_text.cpp
   # file generated by running compiler on samples/monster.fbs
   ${CMAKE_CURRENT_BINARY_DIR}/samples/monster_generated.h