[PATCH modular 1/8] build.sh: expose the PREFIX variable for the user to set

[Trevor Woerner]

I'm just wondering how you would update the wiki page to describe how
to use the script?: "...currently the build.sh script requires 1
argument to be supplied on the cmdline, unless PREFIX is already
defined in the environment in which case you must not provide the
required argument because it would be supplied twice which causes the
script to fail."? In other words, after applying this patch the usage
becomes confusing since the prefix can't be supplied twice.

Also, this patch implies that the default PREFIX is /usr/local, which
sort of implies the user doesn't need to provide it. But if the user
doesn't supply a prefix or it isn't defined in the environment then
the script fails. If the user does provide the prefix (either on the
cmdline or in the environment (but not both!)) then it is whatever the
user provides, not necessarily /usr/local.