Fix Unix build using clang with g++ 4.8 headers.

Don't override HAVE_TR1_TYPE_TRAITS detected by configure with
HAVE_TYPE_TRAITS detected by clang __has_include() as the latter is less
reliable and can return true even when the header is not actually usable, as
it happens with g++ 4.8 <type_traits> in non C++11 mode.

Closes #15915.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@75983 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Fix harmless warnings about unused global variables.

Don't define variables that we never use to fix clang warnings about them.

See #15915.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@75984 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775