configure: Fix alignment tests when cross compiling
When inserting custom code to AC_CHECK_ALIGNOF, make sure to not replace the default includes, but instead append to them. This fixes ALIGNOF_GUINT32 and ALIGNOF_GUINT64 that were both 0 when cross compiling. The third 'unsigned long' test wasn't affected because the AC_CHECK_ALIGNOF call didn't specify the optional 2nd parameter.
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