I18n: Update translation pt (100%).

207 translated messages.

Transifex (https://www.transifex.com/projects/p/xfce/).
Make conditional checks conditional (Bug 9243)

The configure.ac.in script uses AC_CHECK_HEADERS for a ton of
things but the source files don't use those conditionals. This
patch adds those. This should fix building on GNU/Hurd.