This works the same way as LLVM_PARALLEL_COMPILE_JOBS except that it is
specific to the flang source rather than for the whole project.
Configuring with -DFLANG_PARALLEL_COMPILE_JOBS=1 would mean that there
would only ever be one flang source being compiled at a time.
Some of the flang sources require large amounts of memory to compile, so
this option can be used to avoid OOM erros when compiling those files
while still allowing the rest of the project to compile using the
maximum number of jobs.
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Co-authored-by: Nikita Popov <github@npopov.com>
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