Add doc for wxFlexGridSizer::GetRowHeights and GetColWidths

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75579 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Allow building wxGTK using GTK+3 under Windows too.

No real changes, just use the right libraries for GTK+3 port.

Closes #15871.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75585 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775