Adapt the build system to handle Qt4 and Qt5

The library name for Qt4 is sysstat, for Qt5 is sysstat-qt5.
The headers for Qt4 are:
#include <sysstat/version.h> and #include <SysStat/Version>
The headers for Qt5 are:
#include <sysstat-qt5.version.h> and #include <SysStat/Version>

New code should use the portable way:
#include <SysStat/Version>
as it works with Qt4 and Qt5.

Signed-off-by: Luís Pereira <luis.artur.pereira@gmail.com>
Use QString::fromLatin1() instead of QString::fromAscii()

QString::fromAscii() is obsolete in Qt5.

Signed-off-by: Luís Pereira <luis.artur.pereira@gmail.com>