This is just a dotest check to see if we can compile a simple program that uses libc++. Right now we are
parsing the rather big algorithm header in the test program, but the test really just checks
whether we can find *any* libc++ headers and link against some libc++ SO. Using the much smaller
cassert header for checking whether we can find libc++ headers speeds up this check by a bit.
After some incredibly unscientific performance testing this saves a few seconds when running the
test suite on Linux (on macOS we hardcoded that libc++ is always there, so this check won't be
used there and we don't save any time).