Add an LLVM_GENERATOR variable to pick the CMake generator for the LLVM
builds. Default to Ninja, which builds the large LLVM tree faster; set
LLVM_GENERATOR="Unix Makefiles" to use make instead.

README: add ninja and cmake to the prerequisite install commands for
Arch Linux, Fedora/CentOS/RHEL, and macOS, as both are required for
LLVM builds now that Ninja is the default generator.