Add rpath for PG_LIBDIR.

[esabouraud]

This PR ensures that libpq.so is loaded at run time from the same directory as the one used at compile time.
The goal is to remove ambiguity at run time on systems where multiple versions of the PostgreSQL client are installed. Of course this is however less flexible because multiple versions of pgqd have to be built against different versions of libpq, but this protects against unforeseen API/ABI breakages.

Before:

# readelf -d pgqd

Dynamic section at offset 0xddf0 contains 26 entries:
  Tag        Type                         Name/Value
 0x0000000000000001 (NEEDED)             Shared library: [libpq.so.5]
 0x0000000000000001 (NEEDED)             Shared library: [libevent-2.1.so.6]
 0x0000000000000001 (NEEDED)             Shared library: [libc.so.6]
 ...

After (built on RHEL8, linking against PostgreSQL 13):

# readelf -d pgqd

Dynamic section at offset 0xdde0 contains 27 entries:
  Tag        Type                         Name/Value
 0x0000000000000001 (NEEDED)             Shared library: [libpq.so.5]
 0x0000000000000001 (NEEDED)             Shared library: [libevent-2.1.so.6]
 0x0000000000000001 (NEEDED)             Shared library: [libc.so.6]
 0x000000000000000f (RPATH)              Library rpath: [/usr/pgsql-13/lib]
 ...