need pkgconfig expert to add a 'Requires: qhullstatic_r' line

[cbbarber]

[Issue requested by spacelm #128 ]

qhullcpp.pc is broken and should be generated with an extra 'Requires: qhullstatic_r' line. See the current implementation at or near CMakeLists.txt, line 751, 'foreach(pkgconfig in items ...'

For pkg-config documentation, well I know https://people.freedesktop.org/~dbn/pkg-config-guide.html#writing. It's more tedious to implement with current logic, because we have to inject an extra field for a specific .pc file, since currently there is just a loop with common logic based on template https://github.com/qhull/qhull/blob/c2ef2209c28dc61ccfd22514971236587e820121/build/qhull.pc.in for each .pc file.