configure: do not use '@' in version string

#0a0410dd0cb0 broke #0cc4d6352c3e. This patch fixes this, which fixes the build
of gcc/glibc/gdb manuals.

Signed-off-by: "Benoît Thébaudeau" <benoit.thebaudeau@advansee.com>

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