Useful options:

https://www.llvm.org/docs/CMake.html

Use ninja instead of make, it's faster.

LLVM_PARALLEL_LINK_JOBS:STRING
    Define the maximum number of concurrent link jobs.

=> (Static) link is very memory-intensive, you can't have multiple
link jobs in parallel without >32Gb of RAM. This variable helps
running "make -j 4" (or ninja, seems to do it by default) but keep
only one parallel link.


BUILD_SHARED_LIBS:BOOL
    Flag indicating if each LLVM component (e.g. Support) is built as
    a shared library (ON) or as a static library (OFF). Its default
    value is OFF. On Windows, shared libraries may be used when
    building with MinGW, including mingw-w64, but not when building
    with the Microsoft toolchain.

=> Static link is slow, dynamic link is much faster.

LLVM_CCACHE_BUILD:BOOL
    If enabled and the ccache program is available, then LLVM will be
    built using ccache to speed up rebuilds of LLVM and its
    components. Defaults to OFF. The size and location of the cache
    maintained by ccache can be adjusted via the LLVM_CCACHE_MAXSIZE
    and LLVM_CCACHE_DIR options, which are passed to the
    CCACHE_MAXSIZE and CCACHE_DIR environment variables, respectively.

=> Can help to avoid recompiling stuff all the time.