usleep() is apparently deprecated. This change also fixes a compiler
error on GCC when using -std=c99.

Fix issue #53.

Fix issue #55.

This issue is related to how strings for select backends are not deeply
copied, but rather only have their pointers copied. This results in a
situation where the client may delete the strings they passed to the
context config assuming miniaudio makes a copy of it when in fact it
hasn't.