The `test` builtin/binary usually accepts `==` for string comparison, it is
mostly accepted for typos and people being used to double equals, but is not
documented and not always accepted either. Since autoconf uses the default
shell, it might just fail in some cases with:
./configure: 29986: test: xrustc: unexpected operator
./configure: 29990: test: xcargo: unexpected operator
Just change it to single equals as it is done everywhere else.
Signed-off-by: Martin Kletzander <mkletzan@redhat.com>