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>

 python

 import sys

 sys.path.insert(0, '@@PREFIX@@/share/gcc-@@VERSION@@/python')

 from libstdcxx.v6.printers import register_libstdcxx_printers

 register_libstdcxx_printers (None)

 end