While cross compiling, assume the gcc signbit support for the musl as well

[Necktwi Ozfguah]

Please patch the gnulib with 
https://u7535577.ct.sendgrid.net/wf/click?upn=S8VPHvg5-2FzxqlLfWXPq42mUxcZDSyC3MqblYskqrUpUxEeXj2ty4PFy2n-2BuTk72hD1MBwWBQhPZQm9-2FjzWfaTam8LAKZgRnJaqW2A7Bwxgbx-2FknGWQJOWHXuaHJTvXOh_xXjksHEkT5Azm40kLn-2F-2BTLbhdQ6JTRXNvWHWgEemmRmZ9kYxKPUO5OrxTI4CK2U-2FBnPN4EijU2GSGQgUkgXiShfJnYdBm9cj6t1jdaxPPFNCwmHIQ0YZ9WWVz41uf5XAw8alb6q0aFBsq7hYv8-2F-2B12ZuF0NaOBwKsgyeBGUJzd56AQ2Su1zr3FaFuP5-2Ff5WN6YXoSelitN0QT1DRaCVAuQ-3D-3D

... Necktwi