Mojave fixes part 2
Fixes the Makefiles so that compilation works on Mojave, as discussed in #542 (closed). To these ends, introduces a new macos_target
variable which can be set via the environment or the make
command to adjust the target platform (default is 10.9 now, so that compilation works on macOS 10.14 using Xcode 10).
NOTE: This also gets rid of the fftease and lyonpotpourri submodules and adds their sources to the main module, as requested in #542 (closed).