build_options.cmake CFLAGS creation is wrong [minor issue]

[kingtaurus]

The creation of the CFLAGS as found in src/build_options.cmake is wrong. Since compliers often language specific flags (for example -std=c++98 or -std=c++11). Trying test builds with a language specific flag (-std=c++98) will cause clang to throw an error when attempting to compile src/voglcore/rmalloc.c and other pieces of C code.

Note: gcc just gives a warning