build: add --local-flint to set rpath in dev build

[oscarbenjamin]

This implements https://github.com/mesonbuild/meson/issues/13046#issuecomment-2099453043.

On Linux the current meson build needs LD_LIBRARY_PATH to be set when using a local build of flint. This adds a project option to install the rpaths and also customises spin's build command so that you can do:

spin build --local-flint=.local/lib/pkgconfig