Explicitly declare environ variable.

Explicitly declare environ variable.

While POSIX and SUS both mandate this variable existence, they don't say that
it needs to be declared anywhere and it actually doesn't seem to be declared
under Solaris when using Sun CC and so using it resulted in compilation errors.

Just declare it ourselves to avoid them.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@69564 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775