Fixed compiler warning

I moved the constant to the enum in order to avoid this:
ext/pgsql/pgsql.c:3491: warning: comparison is always false due to limited range of data type

Works in gcc 4.6, but I'm not sure about other compilers.
Fixed other compiler warnings in PDO_PGSQL
Fixed compiler warnings in ext/pgsql
Merge branch 'PHP-5.4' into PHP-5.5

* PHP-5.4:
  Fixed compiler warnings in ext/pgsql
  Fixed other compiler warnings in PDO_PGSQL
  Fixed compiler warning