If there are multiple versions of nasm in the PATH, try them all

[kornelski]

Xcode's build environment inserts macOS SDK binaries as first in the path, and Apple ships an ancient unusable version of nasm.
This effectively breaks nasm-based builds, even on machines that have a good copy of nasm installed via homebrew. It's a PITA issue to debug when the same build command works in terminal, but not in Xcode's shell script.

This works around the issue by trying normal "nasm" first, as before. If that nasm is too old, then searches rest of the PATH. Uses std to parse PATH in a portable way.

It also fixes error message in case no nasm is found. Previously it'd print "This version of NASM is too old: No such file or directory". Now it handles Command's error explicitly with a better message.

[medek]

Okay, but you can just delete the xcode shipped nasm and nothing will break.