[configure] remove useless variable declarations

`configure_tool` performs everything by itself.

configure.in

@@ -657,14 +657,6 @@ REQUIRE_LABLGTK=
 USE_LABLGTK=
 HAS_LABLGTK=
 
-# Tool declarations
-####################
-
-DOT=
-REQUIRE_DOT=
-USE_DOT=
-HAS_DOT=
-
 ### Now plugin declarations
 
 PLUGINS_FORCE_LIST=
@@ -974,8 +966,6 @@ EXTERNAL_PLUGINS="${EXTERNAL_PLUGINS} ${EXTRA_EXTERNAL_PLUGINS}"
 AC_SUBST(PLATFORM)
 AC_SUBST(VERBOSEMAKE)
 AC_SUBST(DEVELOPMENT)
-AC_SUBST(DOT)
-AC_SUBST(HAS_DOT)
 AC_SUBST(HAS_APRON)
 AC_SUBST(HAS_MPFR)
 AC_SUBST(HAS_LANDMARKS)