[r22892][Bug #697402] configure doesn't find python on Debian

Lose the PKG_CHECK_MODULES for python and make sure that Python.h is
present and findable in AC_PYTHON_DEVEL instead.

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/branches/2.4@22893 57a11ea4-9604-0410-9ed3-97b8803252fd