Make pulse compile with clang

Make pulse compile with clang

This fixes the checking of supported compiler flags and the following error message for svolume_mmx:

pulsecore/svolume_mmx.c:157:76: error: invalid use of a cast in a inline asm context requiring an l-value:
  remove the cast or build with -fheinous-gnu-extensions
        : "+r" (samples), "+r" (volumes), "+r" (length), "=D" ((pa_reg_x86)channel), "=&r" (temp)
                                                               ~~~~~~~~~~~~^~~~~~~