configure: Create "doc" directory in build directory.

Otherwise we get build failures from the autobuilder when the rule
in gui/automake.mk tries to create doc/help-pages-list.  Problem
introduced in commit 531de52e961c (PsppireDialog: New property:
help-page).

Developers probably didn't notice this problem because they probably
already had doc/ directories in their build directories due to
building PDF versions of the documentation at one time or another, or
because they do builds in the source directory.

Found by examining autobuilder failures.