Added --prefix-include option for generated includes.

Change-Id: I288cda33345362f9aec5cbe309e590bf64f328e1
Tested: on Linux.

docs/source/Compiler.md

@@ -118,5 +118,8 @@ Additional options:
     an evolution of. Gives errors if not. Useful to check if schema
     modifications don't break schema evolution rules.
 
+-   `--include-prefix PATH` : Prefix this path to any generated include
+    statements.
+
 NOTE: short-form options for generators are deprecated, use the long form
 whenever possible.

include/flatbuffers/idl.h

@@ -355,6 +355,7 @@ struct IDLOptions {
   std::string cpp_object_api_pointer_type;
   bool union_value_namespacing;
   bool allow_non_utf8;
+  std::string include_prefix;
 
   // Possible options for the more general generator below.
   enum Language {

src/flatc.cpp

@@ -97,6 +97,8 @@ std::string FlatCompiler::GetUsageString(const char* program_name) const {
       "                     an evolution of. Gives errors if not.\n"
       "  --conform-includes Include path for the schema given with --conform\n"
       "    PATH             \n"
+      "  --include-prefix   Prefix this path to any generated include statements.\n"
+      "    PATH\n"
       "FILEs may be schemas, or JSON files (conforming to preceding schema)\n"
       "FILEs after the -- must be binary flatbuffer format files.\n"
       "Output files are named using the base file name of the input,\n"
@@ -142,6 +144,11 @@ int FlatCompiler::Compile(int argc, const char** argv) {
       } else if (arg == "--conform-includes") {
         if (++argi >= argc) Error("missing path following" + arg, true);
         conform_include_directories.push_back(argv[argi]);
+      } else if (arg == "--include-prefix") {
+        if (++argi >= argc) Error("missing path following" + arg, true);
+        opts.include_prefix = argv[argi];
+        if (opts.include_prefix.back() != '/' &&
+            opts.include_prefix.back() != '\\') opts.include_prefix += "/";
       } else if(arg == "--strict-json") {
         opts.strict_json = true;
       } else if(arg == "--allow-non-utf8") {

src/idl_gen_cpp.cpp

@@ -70,7 +70,8 @@ class CppGenerator : public BaseGenerator {
       const auto basename =
           flatbuffers::StripPath(flatbuffers::StripExtension(it->first));
       if (basename != file_name_) {
-        code_ += "#include \"" + basename + "_generated.h\"";
+        code_ += "#include \"" + parser_.opts.include_prefix + basename +
+                 "_generated.h\"";
         num_includes++;
       }
     }