Use -DPREFIX instead of --prefix _ on OS X

The --prefix parameter differs between yasm and nasm; nasm
requires --prefix _ while yasm requires --prefix=_. Using the
define instead (as on windows) allows easier switching between
the two otherwise mostly compatible assemblers.

This fixes building with yasm on OS X in 32 bit mode. For 64 bit,
a few more tweaks are still required.
Merge pull request #508 from mstorsjo/fix-yasm-osx

Use -DPREFIX instead of --prefix _ on OS X