Install with relocatable enabled fails

[francoisk]

When installing libicuuc with config.install.relocatable=true I get the following error:

b install: config.install.root=/tmp/installed config.install.relocatable=true libicuuc/
error: relocatable installation requires relative base directory
  info: pass empty relative base directory if this call does not affect installation relocatability
  info: or add `assert (!$install.relocatable) 'relocatable installation not supported'` before the call
  libicuuc/libicu/buildfile:105:18: info: while calling install.resolve(dir_path)
  info: while applying rule install.alias to update (for install) ../icu-cfg/dir{libicuuc/}
info: failed to update (for install) ../icu-cfg/dir{libicuuc/}

Code in question:

icu/libicuuc/libicu/buildfile

Line 105 in 2148e5c

plugins_dir = $install.resolve($install.lib)/icu