Fix harmless but annoying warnings in MSVC 12 build.

Suppress warnings about inconsistent DLL linkage for the functions declared in
Python.h.
Don't pass empty charset to wxFontMapper::CharsetToEncoding().

This results in asserts in debug build and is useless anyhow, an encoded word
without a properly specified encoding is malformed anyhow.
Fix bug preventing URLs in last message line from being highlighted.

Don't compare the beginning of the URL with the start of the next line if
there is no next line. This test always failed for the URLs in the last line
of the message preventing them from being highlighted.
Detect wrapped URLs even if there is a space before the new line.

It may happen that the URL is wrapped not quite at the end of a line but
before some spaces preceding it, still recognize it in this case.
Avoid recognizing a line of dashes as part of a wrapped URL.

This often results in false positives as lines of dashes are often used as
section delimiters in emails (especially automatically-generated ones, such as
those sent by Review Board), but consecutive dashes almost never appear in
URLs, so assume they are not part of it.
Compilation fixes for building with wxWidgets 2.8 compatibility disabled.

Also fix (suppress, really) warnings when building with wxWidgets 3.1-trunk.