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>

# Compute Alpha-specific values

CT_DoArchTupleValues () {

    # The architecture part of the tuple:

    CT_TARGET_ARCH="${CT_ARCH}${CT_ARCH_ALPHA_VARIANT}"

}