Update Cygwin/MinGW build instructions.

Remove very out of date information, mention MinGW-w64 and extend Cygwin
explanations.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75021 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Also add mention of TDM-GCC to the build instructions.

This seems to be a very popular option for building wxWidgets too.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75022 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Mention -std=gnu++11 vs -std=c++11 in the build instructions.

Unfortunately -std=c++11 doesn't work currently and almost certainly won't
before 3.0, so at least document the workaround.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75023 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775