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.

I went with the naive solution first.

From [autoconf documentation](https://www.gnu.org/software/autoconf/manual/autoconf-2.65/html_node/Particular-Functions.html) the proper way is to define as follows:

```C
#ifdef HAVE_ALLOCA_H
# include <alloca.h>
#elif defined __GNUC__
# define alloca __builtin_alloca
#elif defined _AIX
# define alloca __alloca
#elif defined _MSC_VER
# include <malloc.h>
# define alloca _alloca
#else
# include <stddef.h>
# ifdef  __cplusplus
extern "C"
# endif
void *alloca (size_t);
#endif
```

This can be stuffed into a separate header such as `alloca-conf.h` and then included in C files. My autoconf-fu is terribly weak though.