Replace existing WITH_MULTIPROCESS build option which searches for an external
libmultiprocess installation and enables building multiprocess binaries option
with new, separate BUILD_MULTIPROCESS and WITH_LIBMULTIPROCESS options.
The BUILD_MULTIPROCESS option controls whether new multiprocess binaries
bitcoin-node and bitcoin-gui will be built.
The WITH_LIBMULTIPROCESS option controls whether to search for an external
libmultiprocess install. If it is true, the external install will be used,
otherwise the local git subtree src/ipc/libmultiprocess subtree will be used.
This option does not usually need to be used except when cross-compiling.
