Allow the user to define the library search path for gmp.lib

[notdanilo]

This is related to #25

We want to make it easier to the https://github.com/nash-io/openlimits users to use the library without worrying about compiling gmp copying and copying it to the expected path.

I just created a hacky build.rs to automatically copy the gmp.lib and gmp.dll files to the toolchain lib folder, but if the user doesn't want to use openlimits anymore and if the user forget about it, the user will leave both files in the folder.

I just found a reasonable solution for it in this stackoverflow answer https://stackoverflow.com/a/32868075 and I would love to see it implemented here.

[omershlo]

thank you @notdanilo !
we chose to handle this by allowing a native rust support. see https://github.com/ZenGo-X/curv/pull/106/files
that said - if you can make a PR with your idea we will be happy to explore