meson: libpq: Revise static / shared library setup

meson: libpq: Revise static / shared library setup

Improvements:
- we don't need -DFRONTEND for libpq anymore since 1d77afefbd1
- the .pc file contents for a static libpq were wrong (referencing
  {pgport, common}_shlib)
- incidentally fixes meson not supporting link_whole on AIX yet
- added explanatory comments

Previously I tried to avoid building libpq's sources twice, once for the
static and once for the shared library. We could still do so, but it's not
clear that it's worth the complication.

Reviewed-by: Peter Eisentraut <peter.eisentraut@enterprisedb.com>