tests: don't fail configury if libunwind is not available.

test/configure.ac

@@ -197,7 +197,10 @@ dnl libunwind.h when configured), but SDL2_test is a static library, so
 dnl there's no way for it to link against it. We could make SDL2 depend on
 dnl it, but we don't want all SDL2 build to suddenly gain an extra dependency,
 dnl so just assume that if it's here now, SDL2_test was probably built with it.
-PKG_CHECK_MODULES(libunwind, libunwind, [LIBS="$LIBS $libunwind_LIBS"])
+PKG_CHECK_MODULES(LIBUNWIND, libunwind, have_libunwind=yes, have_libunwind=no)
+if test x$have_libunwind = xyes ; then
+   LIBS="$LIBS $LIBUNWIND_LIBS"
+fi
 
 dnl Finally create all the generated files
 AC_CONFIG_FILES([Makefile])