Tweaks for Mac compilation.

[Gamnn]

Allows building with homebrew-installed qwt, and works around QT bug on macOS 10.12 (and possibly others) where ld-analyse wouldn't refresh after pressing frame buttons.

[simoninns]

Seems to me that most of these changes are because QWT and opencv2 aren't installed in the correct place within the build environment. Would it not be a lot neater to correct the build environment rather than the project - especially since there is no way to test building in proprietary OSs, so the chance of regression is very high?

The Qt bug work-arounds are a different thing; that just is as it is.

[atsampson]

Adding -I/usr/include/qwt then doing #include <qwt_whatever.h> is probably the neatest fix - that's what GNURadio does, for example. (While installing Qwt's headers into a subdirectory isn't the default in Qwt's build system, it's the usual practice in GNU/Linux distros to reduce /usr/include pollution - e.g. anything Debian- or CentOS-derived has that layout - so I'd be tempted to suggest that Homebrew should do the same.)

For isnanf, add #include <cmath> and change the code to use isnan instead? It has overloads for float and double in C++ (and in C these days).

It would be a good idea to use git rebase -i and git push -f to squash your updates into your original commit so you don't break bisection.

[Gamnn]

Fixed as @atsampson suggested. I think the homebrew-specific stuff should be okay for now; the paths are no longer version-specific at least. So it shouldn't break. I'd like to keep the mac building instructions simple by just needing to run brew install, and not have to do manual linking.

As an aside, I think there'll still be building issues on some other non-Ubuntu distros, due to Ubuntu's qwt-qt5 naming on it instead of qwt, but I'm uncertain of an easy work-around for that. I've left a comment in the code as a hint though.