Fixes for OS X

Fixes for OS X

 * Add some type casting so tv_usec can really be a long, instead of
   some strange platform specific type.

 * Add some .dylib style files to .gitignore.

 * Switch from using -Xlinker to -Wl,. For [reasons unknown][], newer
   versions of GCC, when compiling the Homebrew formula for Asterisk,
   are not properly passing the -Xlinker options to the linker. Given
   that -Wl, does exactly the [same thing][], and does it properly, this
   patch changes the -Xlinker options to use -Wl, instead.

 [reasons unknown]: http://bit.ly/1SUbEYx
 [same thing]: https://gcc.gnu.org/onlinedocs/gcc/Link-Options.html

Change-Id: Id5e6b3c6cc86282ea5fca630dc3991137c5bf4dd