.c files only need to import psycopg.h: it will in turn import
dependencies from Python and libpq and configure.h. psycopg.h should be
the first to be imported, so the basic imports are not required in
the .h's
As a guideline I'm trying to import from the most specific to the most
generic to detect missing imports in the .h's.
that should not be exported from the module. This results in a 5%
reduction in code size and shortens the dynamic symbol table.
* psycopg/config.h: If GCC >= 4.0 is installed, define the HIDDEN
symbol to apply the "hidden" visibility attribute.
