Remove wxHAVE_GLIBC2 from configure, always predefine _GNU_SOURCE.

Remove wxHAVE_GLIBC2 from configure, always predefine _GNU_SOURCE.

We used to test for glibc version first and predefined _GNU_SOURCE only if it
was greater than 2.1 but there doesn't seem to be any harm to just always
predefine _GNU_SOURCE under Linux (not that there are any systems with glibc <
2.1 left anyhow). Also do it much earlier to ensure that all tests are
affected by it.

And as we don't use wxHAVE_GLIBC2 anywhere else just remove it.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@69935 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775