This is an old convention that hasn't been used by Blender or CMake distributed Find* files for a long time.
gmpxx library (C++ version of gmp) uses symbols from libgmp, which means the libgmp is to be passed to the linker after libgmpxx.
Required for the new boolean code, disabled by default until all platforms have landed the libs and the boolean code actually lands in master. Reviewed By: brecht Differential Revision: https://developer.blender.org/D8384
