build,qt: Explicitly namespace a few Qt variables for Qt5

m4/qt.m4: Rename to
m4/qt5.m4: this.
(FIND_QT): Rename to FIND_QT5.
(GPGME_QT): Change variable prefix to GPGME_QT5.
(GPGME_QTTEST: Change variable prefix to GPGME_QT5TEST.
configure.ac, lang/qt/src/Makefile.am, lang/qt/tests/Makefile.am:

Adjust accordingly.

In preparation to adding support for building qgpgme for Qt6, add the
version number to a few variables to avoid confusion.