configure.ac contains Bashisms

configure.ac contains Bashisms

Autoconf scripts are supposed to be written in POSIX-compatible shell
language. elfutils commits 0ffe2874e75d48cb88936e119e5284ff2bf664d9
and 3fa98a6f29b0f370e32549ead7eb897c839af980 introduced Bashisms to
configure.ac that cause errors when /bin/sh is not Bash.

Example error when /bin/sh is Dash:

./configure: 8748: test: xyes: unexpected operator

The cause of the error is the use of a non-existent == operator in
expressions passed to the 'test' built-in command. POSIX shell
specifies only an = operator for testing string equality.

  * configure.ac: Replace == by = in use_msan and enable_demangler
  comparisons.

https://sourceware.org/bugzilla/show_bug.cgi?id=30196

Reported-by: Matt Whitlock <sourceware@mattwhitlock.name>
Signed-off-by: Mark Wielaard <mark@klomp.org>