If available, use qtchooser to select correct Qt tools

If available, use qtchooser to select correct Qt tools

When the Qt directory is specified by --with-qt-dir, the tools
are only searched for in the corresponding bin subdir and their
full path is retained, so that it is not necessary modifying the
PATH and PKG_CONFIG_PATH environment variables to select a custom
Qt version. Otherwise, if qtchooser is detected early in the PATH,
the generic names are attempted with proper arguments to select the
desired Qt version (e.g., "moc -qt=qt5" if --enable-qt5 is specified).
Failing all of the above, the generic names with and without proper
suffixes (either -qt4 or -qt5) are checked in the PATH.
Eventually, a check is performed and a warning is issued if the
selected tools do not match the chosen Qt libraries.