Add a check for the Darwin dynamic linker. Use AC_TRY_LINK when checking

Add a check for the Darwin dynamic linker. Use AC_TRY_LINK when checking

        * configure.in: Add a check for the Darwin dynamic linker. Use
        AC_TRY_LINK when checking for "nonposix getpwuid_r" so it notices
        "no getpwuid_r" correctly.

        * testglib.c (main): Make template[] bigger to prevent an overrun.
        Remove an unused variable. Initialize error to NULL.

        * tests/gio-test.c (main): Add a cast to prevent a warning when
        size_t is a long.

        * tests/type-test.c (main): Add an #ifdef to prevent a warning
        when G_HAVE_GINT64 is defined and G_GINT64_FORMAT isn't.