configure: have bash_absolute and perl_absolute always defined

Since set -u is used, without bash or perl, configure would fail.

This has gone unnoticed as (almost) everyone always had both
bash and perl installed (and in $PATH).

Thanks to FreeBSD ports this bug became visible; this change is
verbatim copy of `patch-configure` in FreeBSD ports tree.

configure

@@ -734,6 +734,7 @@ if command -v ${BASHCMD} > /dev/null; then
     printf "Yes (%s).\n" "$bash_absolute"
 else
     have_bash=0
+    bash_absolute=
     printf "No. (%s not found)\n" "${BASHCMD}"
 fi
 
@@ -744,6 +745,7 @@ if command -v ${PERL} > /dev/null; then
     printf "Yes (%s).\n" "$perl_absolute"
 else
     have_perl=0
+    perl_absolute=
     printf "No. (%s not found)\n" "${PERL}"
 fi