Simplify and fix column determination code in wxGridStringTable.

The number of columns in the table is now (since r60717) stored in m_numCols,
so there is no need to get it from wxGrid any more. This is not only simpler
and faster but also more correct as the first row might not have the full
number of values yet.

Closes #15809.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75552 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Revert "Define wxUSE_UNICODE on compiler command line in Unix builds too."

This fix only helps with building wxWidgets itself but doesn't help when
building user code using it, so revert it as we need another solution to this
problem anyhow.

This reverts r75536, see #15805.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75553 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Revert "Move UNICODE section of wx/platform.h before wx/compiler.h inclusion."

Unfortunately this fix for MinGW broke compilation under Unix, see #15805.

This reverts r75520.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75554 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Move wxCHECK_MINGW32_VERSION() declaration to wx/msw/gccpriv.h.

Unfortunately we can't keep it in wx/compiler.h which is included too early,
before UNICODE macro is defined, and we can't define it before including
wx/setup.h which itself must be included after wx/compiler.h.

So move it to wx/msw/gccpriv.h which is not a great solution, as the best
would be to get rid of this gcc-specific file completely, but at least should
make everything build again and ensure that _mingw.h is included after UNICODE
definition.

Closes #15805.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75555 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Remove unnecessary _mingw.h inclusion from wx/msw/missing.h.

This header is already included from wx/platform.h which is always included
before wx/msw/missing.h.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75556 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775