[OCaml] Allow out-of-tree builds of LLVM bindings.

In order to use this feature, configure LLVM as usual,
but then build and install it as:

   make all install SYSTEM_LLVM_CONFIG=llvm-config

where llvm-config is the llvm-config binary installed on your
system (possibly llvm-config-VERSION on e.g. Debian).

llvm-svn: 225787