changing GL_CXX_FLAGS to append rather than overrite CXX_FLAGS
changing GL_CXX_FLAGS to append rather than overrite CXX_FLAGS

danieldresser/cortex,goddardl/cortex,danieldresser/cortex,lento/cortex,davidsminor/cortex,davidsminor/cortex,appleseedhq/cortex,goddardl/cortex,hradec/cortex,davidsminor/cortex,lento/cortex,appleseedhq/cortex,hradec/cortex,danieldresser/cortex,davidsminor/cortex,goddardl/cortex,appleseedhq/cortex,danieldresser/cortex,lento/cortex,hradec/cortex