Request: Allow sysext-build on other prefixes than /usr

Right now, sysext-build seems a bit hardcoded into thinking that packages will be installed into /usr. It would be nice to allow e.g. passing a --prefix argument so other prefixes can be used. This would alter the paths for e.g.:

• Glib compiled schemas
• Icon caches
• Etc

In order to support a scheme that relies on other observed paths (/usr/local, mainly).