Provide special pseudo-macro %{macrosfile}

     Change: expand-macrosfile-macro
     Purpose: Provide special pseudo-macro %{macrosfile}
     Reason: In order to support local .rpmmacros (those
                     which stay directly inside a development area)
                     which redefine %{_specdir}/%{_sourcedir}/etc to
                     point to local directories, it is necessary to be
                     able to override those macros with absolute paths
                     relative to the .rpmmacros file itself. For this
                     it is necessary that one can expand a macro which
                     results in the pathname of the .rpmmacros file
                     itself. Together with %{dirname:...} and other
                     macro constructs this way one can allow arbitrarily
                     relocatable development environments.