add isblank() equivalent and use it instead of checking for (tab or

add isblank() equivalent and use it instead of checking for (tab or
space) all over the code, properly detect whether we have strcasestr()
and only use nstristr() if we don't, and bump up CVS build requirements
to account for setting _GNU_SOURCE when running the test programs for
both

git-svn-id: svn://svn.savannah.gnu.org/nano/trunk/nano@1730 35c25a1d-7b9e-4130-9fde-d3aeb78583b8