Clarification questions on BOOST_ROOT in ax_boost_base.m4

It looks like the Boost macro prioritizes /usr, /usr/local, /opt, /opt/local ahead of $BOOST_ROOT. In other words, hard-coded directory paths take precedence over an environment variable. I was surprised by this choice. Is it intentional?

Similarly, the $BOOST_ROOT must have a "stage" in it, which means I can’t just set $BOOST_ROOT to the location of my local installation. CMake takes this environment variable as is.

I was able to get around both issues by setting --with-boost=$BOOST_ROOT when running configure. I am just seeking clarification as to whether this is expected behavior.

Thanks,
Chris Aycock