Clarify dependencies

[ceheitsch]

Have libomp (11.0.1) installed and cmake (3.19.4). Cmake is being treated "as a formula". There's no GUI here, so doesn't seem to be a need for a cask. If it matters, please clarify.

llvm (11.0.1) is also now installed. However, there were several caveats:

To use the bundled libc++ please add the following LDFLAGS:
LDFLAGS="-L/usr/local/opt/llvm/lib -Wl,-rpath,/usr/local/opt/llvm/lib"

llvm is keg-only, which means it was not symlinked into /usr/local,
because macOS already provides this software and installing another version in
parallel can cause all kinds of trouble.

If you need to have llvm first in your PATH, run:
echo 'export PATH="/usr/local/opt/llvm/bin:$PATH"' >> ~/.profile

For compilers to find llvm you may need to set:
export LDFLAGS="-L/usr/local/opt/llvm/lib"
export CPPFLAGS="-I/usr/local/opt/llvm/include"

Please clarify which of these need to be addressed.

[ceheitsch]

I followed the "If you need..." directions and also the "For compilers...", but there's still some issue; see attached command line output.
output.txt