ExtUtils::Embed: silence test warning

From the added code comments:

    XXX DAPM 12/2014: ExtUtils::Embed doesn't seem to provide API access
    to $Config{optimize} and so compiles the test code without
    optimisation on optimised perls. This causes the compiler to warn
    when -D_FORTIFY_SOURCE is in force without -O. For now, just strip
    the fortify on optimised builds to avoid the warning.