add ffi and install.libs.R

[GabrielHoffman]

I'm writing a R package with an Rcpp package, and I'd like to access the plink code at the C++ level. You made some small changes before based on this (#283), and here I make two small changes so C++ code in other packages can access plink code at compile time.

1. Modify Makevars.in to include pvar_ffi_support.o and pgenlib_ffi_support.o in libPLINK2.a

2. Add install.libs.R to put static libraries in the default install path, so C++ in other packages can link against them.

In testing, this works on OSX and linux