Fixed minor portability issues to support FreeBSD.

Added support so sniproxy compiles in FreeBSD.

Following issues arose when compiling:
 * `alloca.h` doesn't exist (`alloca()` is defined in `stdlib.h`).
 * `tests/binder_test.c` needs to include `sys/types.h` since it uses funky typedefs when defining constants such as `INADDR_LOOPBACK`.
 * autoconf/pkg-config doesn't set the `HAVE_LIB{EV,UDNS,RT,PCRE}` preprocessor macros in `src/Makefile.in` so I added some minor tweaks to `configure.ac`.

`make check` passes for everything but reload configuration, which to my understanding is not yet supported.

If this is accepted, I will try to add a port to FreeBSD in the next versioned release.

Thanks.

Testing on OpenBSD, it looks like HAVE_ALLOCA_H is the way to go here.