This is an attempt to address the issue of switching from Qt 5 to
Qt 6 or vice versa on Unix without running 'make spotless'. Various
*.h files (but not all of them) in win/Qt/ are run through a program
called 'moc' to create new files *.moc that are included by *.cpp.
The problem is that the constructed files use the same names for Qt
5 or 6 but their contents apparently aren't compatible. This adds
rules (specific to GNU make) that use a pair of timestamp files to
track which version of moc made src/*.moc and to destroy them when
switching Qt versions in order to force their re-creation.
As it stands, a hints file which includes hints/include/compiler.370
is now required in order to build the Qt interface with the Unix
Makefiles. There's no change for platforms that don't use those and
I've no idea whether something comparable could be done for macOS
Xcode IDE.
The first time make is run with WANT_WIN_QT=1 after this is in place,
all the .moc files will be rebuilt and the subset of .cpp files which
include them will be recompiled. After that, the .moc files will be
rebuilt as needed--and only as needed--just like any other constructed
files.
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