configure: Avoid '==' bashism

The '==' operator to test is a bashism; the standard way to copmare
strings is '='. This causes dash to complain:

../../configure: 681: test: linux: unexpected operator

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-id: 20220720152631.450903-6-peter.maydell@linaro.org
diff --git a/configure b/configure
index d0e9a51..2c19329 100755
--- a/configure
+++ b/configure
@@ -678,7 +678,7 @@
 
 meson_option_build_array() {
   printf '['
-  (if test "$targetos" == windows; then
+  (if test "$targetos" = windows; then
     IFS=\;
   else
     IFS=: