PCH-less compilation fix.
Explicitly pass the range to wxTextCtrl::SetStyle().

Using -1,-1 is not documented and apparently different wx ports
interpret it (or even implement) differently.
Update AX_CXX_COMPILE_STDCXX_11 to current version.

This version adds check for the 'override' keyword (which Poedit newly
uses) and a few other C++11 constructs.

In practical terms, override-less gcc-4.6 no longer passes the test.