Fix library install_name on OS X

[ryandesign]

The install_name of the library is not correct on OS X. Here's what the library looks like when building field3d 1.3.2 in MacPorts:

$ otool -L /opt/local/var/macports/build/_Users_rschmidt_macports_dports_graphics_field3d/field3d/work/Field3D-1.3.2/install/darwin/m64/release/lib/libField3D.dylib
/opt/local/var/macports/build/_Users_rschmidt_macports_dports_graphics_field3d/field3d/work/Field3D-1.3.2/install/darwin/m64/release/lib/libField3D.dylib:
    /opt/local/var/macports/build/_Users_rschmidt_macports_dports_graphics_field3d/field3d/work/Field3D-1.3.2/install/darwin/m64/release/lib/libField3D.dylib (compatibility version 0.0.0, current version 0.0.0)
    /opt/local/lib/libHalf.6.dylib (compatibility version 7.0.0, current version 7.0.0)
    /opt/local/lib/libIex.6.dylib (compatibility version 7.0.0, current version 7.0.0)
    /opt/local/lib/libImath.6.dylib (compatibility version 7.0.0, current version 7.0.0)
    /opt/local/lib/libz.1.dylib (compatibility version 1.0.0, current version 1.2.7)
    /usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 169.3.0)
    /opt/local/lib/libhdf5.7.dylib (compatibility version 8.0.0, current version 8.3.0)
    /opt/local/lib/libboost_thread-mt.dylib (compatibility version 0.0.0, current version 0.0.0)
    /opt/local/lib/libboost_system-mt.dylib (compatibility version 0.0.0, current version 0.0.0)
    /usr/lib/libstdc++.6.dylib (compatibility version 7.0.0, current version 56.0.0)

/opt/local/var/macports/build/_Users_rschmidt_macports_dports_graphics_field3d/field3d/work/Field3D-1.3.2/install/darwin/m64/release/lib/libField3D.dylib is the location of the library on disk at build time, but that should not be the path baked into the library as its install_name (shown as the first line of output from otool -L); the install_name needs to be the absolute path the library will have after installation, for example in the case of MacPorts /opt/local/lib/libField3D.dylib. In MacPorts we are fixing this by running install_name_tool manually after the build, but it would be better if you fixed it in your build system.

You should also be setting the current_version and compatibility_version of the library as befits your library version numbering scheme.