drush make on local gitweb raw url
So I found the solution : the shell is interpreting the semicolons and cuts the URL so it has to be quoted. Here for example http://sourceforge.net/apps/trac/sourceforge/ticket/9358 it is explained.

my drush cmd should be:

drush make "http://git.mycompany.de/?p=workflow.git;a=blob_plain;f=build-workflow.make;h=170f5329b413bea0efb1ec33bebdc1549403cc3e;hb=HEAD"