configure.ac: fix bashism

Reviewed-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Tobias Nygren <tnn@NetBSD.org>
diff --git a/configure.ac b/configure.ac
index 6ccf3b4..325b936 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1641,7 +1641,7 @@
     if ! echo "$with_gallium_drivers" | grep -q 'swrast'; then
         AC_MSG_ERROR([nine requires the gallium swrast driver])
     fi
-    if test "x$with_gallium_drivers" == xswrast; then
+    if test "x$with_gallium_drivers" = xswrast; then
         AC_MSG_ERROR([nine requires at least one non-swrast gallium driver])
     fi
     if test "x$enable_dri3" = xno; then