Issues on Mac OS about rqt_multiplot_plugin

[mikepurvis]

I do have a symlink for the framework:

$ ls -la /usr/local/lib/qwt.framework
lrwxr-xr-x  1 mikepurvis  admin  37 15 Jun 22:16 /usr/local/lib/qwt.framework -> ../Cellar/qwt/6.1.3/lib/qwt.framework

It looks like find_package/find_library do search framework paths on OS X, eg:

https://cmake.org/cmake/help/v3.0/command/find_package.html

So the gap would be filled if qwt provided a CMake module. Since it doesn't I think the easiest resolution might be to find the library first, then use the path of the library as the hint for find_path to look for headers.