graphics/py-openexr: fix build on GCC architectures

Use C++11 compiler:
/usr/local/include/OpenEXR/half.h:99: error: invalid pure specifier (only `= 0' is allowed) before ';' token
/usr/local/include/OpenEXR/half.h:102: error: invalid pure specifier (only `= 0' is allowed) before ';' token
/usr/local/include/OpenEXR/half.h:103: error: invalid pure specifier (only `= 0' is allowed) before ';' token
/usr/local/include/OpenEXR/half.h:104: error: expected ',' or '...' before '&&' token
/usr/local/include/OpenEXR/half.h:104: error: invalid constructor; you probably meant 'half (const half&)'
/usr/local/include/OpenEXR/half.h:104: error: expected ';' before 'noexcept'
/usr/local/include/OpenEXR/half.h:124: error: invalid pure specifier (only `= 0' is allowed) before ';' token
/usr/local/include/OpenEXR/half.h:125: error: expected ',' or '...' before '&&' token
/usr/local/include/OpenEXR/half.h:125: error: expected ';' before 'noexcept'
/usr/local/include/OpenEXR/half.h:99: error: initializer specified for non-virtual method 'half::half()'
/usr/local/include/OpenEXR/half.h:102: error: initializer specified for non-virtual method 'half::~half()'
/usr/local/include/OpenEXR/half.h:103: error: initializer specified for non-virtual method 'half::half(const half&)'
/usr/local/include/OpenEXR/half.h:124: error: initializer specified for non-virtual method 'half& half::operator=(const half&)'