Remove support for Digital Mars compiler.

This compilers is not being developed any longer since many years, drop
support for it.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76547 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Remove obsolete MSVC version tests.

Don't compare __VISUALC__ with versions 1200 (VC6) and earlier, such tests are
always true or always false now that we don't support VC6 any more, so just
remove them simplifying the code.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76548 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Remove checks for ancient gcc/MinGW versions.

Don't test for wxUSE_NORLANDER_HEADERS, this is always true since many, many
years.

Remove tests for gcc version < 3.3, notably 2.95: the minimal supported gcc
version is 3.4.

Closes #15727.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76549 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775