[configure] Use hg to compute the version string

Now that we have migrated to hg from svn, it does not make sense
to keep using svn tools to compute the version string, so switch
to using mercurial tools (that are much more flexible, BTW).

.version

@@ -1 +1 @@
-svn
+hg

configure

@@ -364,23 +364,15 @@ has_or_abort lib="${ncurses_libs}"                  \
 # If this version is a svn snapshot, try to get the revision number
 # If we can't get the revision number, use date
 case "${VERSION}" in
-    *+svn|svn)
-        has_or_abort prog=svnversion
+    *+hg|hg)
+        has_or_abort prog=hg
         printf "Computing version string... "
-        REVISION="$( LC_ALL=C svnversion )"
+        REVISION="$( hg id -n 2>/dev/null )"
         case "${REVISION}" in
-            exported)
+            "")
                 VERSION="${VERSION}_unknown@$( date +%Y%m%d.%H%M%S )";;
             *)
-                URL="$( LC_ALL=C svn info 2>/dev/null   \
-                                 |egrep 'URL: '         \
-                                 |cut -d ' ' -f 2-      \
-                      )"
-                ROOT="$( LC_ALL=C svn info 2>/dev/null      \
-                         |"${grep}" '^Repository Root: '    \
-                         |cut -d ' ' -f 3-                  \
-                       )"
-                VERSION="${VERSION}${URL#${ROOT}}@${REVISION}"
+                VERSION="${VERSION}:$( hg id -b )@${REVISION%%+}:$( hg id -i )"
                 ;;
         esac
         # Arrange to have no / in the directory name, no need to create an