glmark2: Fix build errors by enabling c++ 11 features

build was failing with errors:
| ../src/image-reader.cpp: In member function 'bool PNGReader::init(const
string&)':
| ../src/image-reader.cpp:123:16: error: 'unique_ptr' in namespace 'std' does
not name a template type
|      const std::unique_ptr<std::istream>
is_ptr(Util::get_resource(filename));
|                 ^
| ../src/image-reader.cpp:124:12: error: 'is_ptr' was not declared in this
scope
|      if (!(*is_ptr)) {
|             ^
| ../src/image-reader.cpp:149:57: error: 'is_ptr' was not declared in this
scope
|      png_set_read_fn(priv_->png, reinterpret_cast<void*>(is_ptr.get()),
|                                                          ^
| ../src/model.cpp: In member function 'bool Model::load_3ds(const string&)':
| ../src/model.cpp:364:16: error: 'unique_ptr' in namespace 'std' does not name
a template type
|      const std::unique_ptr<std::istream>
input_file_ptr(Util::get_resource(filename));
|                 ^
| ../src/model.cpp:365:31: error: 'input_file_ptr' was not declared in this
scope
|      std::istream& input_file(*input_file_ptr);
|                                ^
| ../src/model.cpp: In member function 'bool Model::load_obj(const string&)':
| ../src/model.cpp:684:16: error: 'unique_ptr' in namespace 'std' does not name
a template type
|      const std::unique_ptr<std::istream>
input_file_ptr(Util::get_resource(filename));
|                 ^
| ../src/model.cpp:685:30: error: 'input_file_ptr' was not declared in this
scope
|      std::istream& inputFile(*input_file_ptr);

Signed-off-by: Amarnath Valluri <amarnath.valluri@intel.com>

Author: avalluri

meta-ostro-xt/recipes-benchmark/glmark2/glmark2_%.bbappend

@@ -0,0 +1,2 @@
+# Enable C++11 features
+CXXFLAGS += "--std=c++11"