Fix build with Sun CC under Linux.

Fix build with Sun CC under Linux.

We need to explicitly define _GNU_SOURCE when building our code as we use it
configure when running the tests and it's not predefined by Sun CC, unlike g++
so we need to add it to CXXFLAGS ourselves.

An alternative solution would be to run the tests without _GNU_SOURCE but this
would lose too much functionality so it doesn't seem like a good idea.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@69936 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775