please use pkg-config to find zlib and OpenSSL

Looking for libraries and their headers in a static list of folders is wrong and tends to break in todays multi-arch environments where people have more than one OpenSSL or libfoo installed.

This patchset first converts the configure script and makefiles to use `pkg-config` (using the macros provided in `pkg.m4`) and then removes all those unused files in the `m4` folder.

The only feature "missing" now, is the ability to say `--with-ssl=/funny/path`. The good side: it did not work in multi-arch environments anyways (e.g. Debian has libs in `/usr/lib/x86_64-linux-gnu`, but --with-ssl insisted on adding `/lib` to the path you gave it). The other good side: you can achieve the proper behaviour by passing `SSL_CFLAGS` and `SSL_LDFLAGS` in the environment calling `configure`.