configure: do not use '@' in version string

Using an '@' in the version string breaks the gcc/glibc/gdb manuals.
Use a scheme similar to Debian, with a plus '+' instead of the '@',
as suggested by Michael Hope.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

configure

@@ -554,7 +554,7 @@ else
                               2>/dev/null                               \
                        || true                                          \
                      )"
-            VERSION="${VERSION}@${rev_id:-unknown-$( date +%Y%m%d.%H%M%S )}"
+            VERSION="${VERSION}+${rev_id:-unknown-$( date +%Y%m%d.%H%M%S )}"
             ;;
     esac
 fi