I suppose you have thousands of possibilities to do that. I don't know what -frandom-seed exactly do, but definitely it's much better to use c++11 random functions. And init the random generator with the seed of your choice.
To solve your situation you can (what I usually do) set the seed as a parameter at execution time.
If you still want to have it at compilation time, you can easily use MY_SEED in the code and add in the compilation command -DMY_SEED=your_favorite_seed_value.
Why not use a series of specific name spaces as opposed to letting the compiler generate one?