minicargo.mk - Allow using a system LLVM
minicargo.mk - Allow using a system LLVM

We only need to build LLVM when using an in-tree llvm-config.
When a system LLVM installation is specified by overriding
LLVM_CONFIG, nothing needs to be built.

thepowersgang/mrustc,thepowersgang/mrustc,thepowersgang/mrustc,thepowersgang/mrustc,thepowersgang/mrustc