configure.ac: avoid bashisms
configure.ac: avoid bashisms

configure scripts need to be written in POSIX-compliant shell; using
'==' is a Bashism (doesn't work for POSIX-compliant shells). Let's
use '=' as we do elsewhere to fix compatibility.

Fixes: #313
Signed-off-by: Sam James <f16bed56189e249fe4ca8ed10a1ecae60e8ceac0@gentoo.org>

libunwind/libunwind,vtjnash/libunwind,libunwind/libunwind,vtjnash/libunwind,frida/libunwind,vtjnash/libunwind,libunwind/libunwind,frida/libunwind,frida/libunwind