Allow vim-qt to build against Qt5

With this commit vim-Qt can now build against Qt5, this
does not fix the input issue identified in issue#56
and with an ongoing fix in issue#96. What it does is

- Fix wrong byte count w/ converting keyboard input
- Replace ASCII conversions with Latin string conversions

By default autoconf will not look for the qmake-qt5 alias,
if you really want to build against Qt5 you need to pass
--with-qt-qmake=/path/to/qmake to autoconf.