The build system is fragile by allowing multiple invocation of
subprocess builds in the native folder for Make generator.
For example, during sub-invocation of the native llvm-config,
llvm-min-tblgen is also built. If there is another sub-invocation of the
native llvm-min-tblgen build running in parallel, they may overwrite
each other's build results, and may lead to errors like "Text file
busy".
This patch adds a cmake script that uses file lock to serialize all
native builds for Make generator.
