feat(build): improve dependency check and error message in autogen.sh

[epiccurious]

Three changes:

1. Improve error reporting in autogen.sh by redirecting the autoreconf dependency check's error message to stderr.
2. Use command -v instead of which in the script.
3. Replace legacy backtick substitution with modern $(...) command substitution.

Impact

This PR modifies autogen.sh in the root directory.

• No change to the functional behavior of these files.
• Enhances error reporting by directing error messages to stderr, aligning with best practices.
• Improves portability/reliability by using command -v instead of which. (Link to ShellCheck rule for context.)
• Improves portability/reliability by using modern $(...) command substitution. (Link to Shellcheck rule for context.)

Background

5 years ago: Created the entire file with no other updates.

• Link to commit.