In some environments, installing the library to GCC's
multi-os-directory is not perferable. This patch adds a switch
to configure "--disable-multi-os-directory" which will disable
that check, typically falling back to ${libdir} unless
cross-compiling.
Original patch was written by Stewart Brodie, and was found at
https://sourceware.org/ml/libffi-discuss/2013/msg00144.html
I've just updated it to work with the current version.
