libstdc++: Fix syntax error in libbacktrace configuration

Using == instead of = causes a configuration error with dash as the
shell:

checking whether to build libbacktrace support... /home/devel/building/work/src/gcc-12-20220417/libstdc++-v3/configure: 77471: test: auto: unexpected operator
/home/devel/building/work/src/gcc-12-20220417/libstdc++-v3/configure: 77474: test: auto: unexpected operator
auto

This means we fail to change the value from "auto" to "no" and so this
test passes:
GLIBCXX_CONDITIONAL(ENABLE_BACKTRACE, [test "$enable_libstdcxx_backtrace" != no])

This leads to the libbacktrace directory being included in the build
without being configured properly, and bootstrap fails.

libstdc++-v3/ChangeLog:

	* acinclude.m4 (GLIBCXX_ENABLE_BACKTRACE): Fix shell operators.
	* configure: Regenerate.