Fix build on FreeBSD

[iskunk]

When compiling os.c on FreeBSD, you need to zap _POSIX_C_SOURCE or else you get this:

[  9%] Building C object CMakeFiles/unit_tests.dir/src/os.c.o
In file included from /tmp/libsoundio/src/os.c:74:
/usr/include/sys/event.h:61:2: error: unknown type name 'u_short'; did you mean
      'short'?
        u_short         flags;
        ^
/usr/include/sys/event.h:62:2: error: unknown type name 'u_int'
        u_int           fflags;
        ^
/tmp/libsoundio/src/os.c:701:71: error: use of undeclared identifier
      'MAP_ANONYMOUS'
  ...*address = (char*)mmap(NULL, actual_capacity * 2, PROT_NONE, MAP_ANONYMO...
                                                                  ^
3 errors generated.

Also, the (FreeBSD) system that I tested on had ALSA development libraries installed for some reason. Unfortunately, the libsoundio ALSA backend requires inotify, so...

[ 16%] Building C object CMakeFiles/unit_tests.dir/src/alsa.c.o
/tmp/libsoundio/src/alsa.c:12:10: fatal error: 'sys/inotify.h' file not
      found
#include <sys/inotify.h>
         ^
1 error generated.

And then there was this curiosity:

[  2%] Building C object CMakeFiles/unit_tests.dir/test/unit_tests.c.o
In file included from /tmp/libsoundio/test/unit_tests.c:3:
In file included from /tmp/libsoundio/src/soundio_private.h:24:
In file included from /tmp/libsoundio/src/alsa.h:16:
In file included from /usr/local/include/alsa/asoundlib.h:63:
/usr/local/include/alsa/pcm.h:597:19: error: zero size arrays are an extension
      [-Werror,-Wzero-length-array]
        unsigned int pos[0];    /**< channel position array */
                         ^
1 error generated.

This last one is cleared by -Wno-zero-length-array.

The first two issues are fixed here: libsoundio-fix-freebsd.patch.txt

[andrewrk]

I'd like to merge this. Could you open this patch as a pull request so that I can give you credit for your work?