C11 threads and mutexes

Illustrating the use of C11 threads and mutexes by recursively searching a directory for filenames for educational purposes on Libera IRC channel #c.

Compile

make

Run

./build/xfs search-string my-path [my-path-two] [my-path-three] ...

Test

./build/xfs hello test

or

./build/xfs hello test/a test/b

should yield

test/a/hello
test/b/hello