mkmf does not allow for linking against custom libraries when a system library is present
Hopefully the title is not confusing, but the short story is that mkmf outputs a makefile that first searches the default lib path before searching any user provided lib paths. This is not an issue until one tries to link against an included library whose version is different than a preexisting system library.
The issue cropped up while trying to install the rugged gem (libgit2 wrapper) and a full dialog on the issue can be found on github https://github.com/libgit2/rugged/issues/351.
I was able to fix the issue with the attached patch (https://github.com/muff1nman/ruby/commit/a0c8bc32cfc11e61c5b9703bff243934c6509210)