configure: Fix build on debian by avoiding bashisms in the test command.
diff --git a/configure.ac b/configure.ac
index c6e1e96..9467ff7 100644
--- a/configure.ac
+++ b/configure.ac
@@ -167,7 +167,7 @@
 	      AS_HELP_STRING([--disable-tests],
 			     [do not build unit tests [default=enabled]]),,
 	      enable_tests=yes)
-AM_CONDITIONAL(BUILD_TESTS, test x$enable_tests == xyes)
+AM_CONDITIONAL(BUILD_TESTS, test x$enable_tests = xyes)
 
 if test "x$GCC" = "xyes"; then
 	GCC_CFLAGS="-Wall -g -Wstrict-prototypes -Wmissing-prototypes -fvisibility=hidden"