Add format attribute checking for printf() like functions

Patch by Marco Nenciarini <mnencia@debian.org>
fix: added forgotten options.priority value

Patch by Marco Nenciarini <mnencia@debian.org>
fix: make pg_config be settable from outside the makefile

Patch by Marco Nenciarini <mnencia@debian.org>
split install target into install_prog and install_ext

Patch by Marco Nenciarini <mnencia@debian.org>