This code:

    #define CSR_FLUSH_TO_ZERO         (1 << 15)
    unsigned csr = __builtin_ia32_stmxcsr();
    csr |= CSR_FLUSH_TO_ZERO;
    __builtin_ia32_ldmxcsr(csr);

from https://stackoverflow.com/a/8217313/ seems to work for me with gcc. I don't know how widely supported it would be.