Avoid macro clash with LLVM 9.

Early previews of LLVM 9 reveal that our Min() macro causes compiler
errors in LLVM headers reached by the #include directives in
llvmjit_inline.cpp.  Let's just undefine it.  Per buildfarm animal
seawasp.  Back-patch to 11.

Reviewed-by: Fabien Coelho, Tom Lane
Discussion: https://postgr.es/m/20190606173216.GA6306%40alvherre.pgsql