gdb: don't use bashism in configure test

gdb: don't use bashism in configure test

Results in configure output like:
```
checking for X... no
/var/tmp/portage/sys-devel/gdb-12.1/work/gdb-12.1/gdb/configure: 18837: test: yes: unexpected operator
checking whether to use babeltrace... auto
```

... when /bin/sh is provided by a POSIX-compliant shell, like dash,
instead of bash.