Build fix

Detail:
  Some users reported problems building the sources if they had other
  installations of perl on their build machine. The build system was using a
  mixture of "perl" and "<Perl$Dir>.perl" to invoke the interpreter, and
  sometimes but not always using "do" to expand system variables on the
  command line. This has now been standardised to use "do <Perl$Dir>.perl in
  all cases, and where possible, to use the makefile macro ${PERL}.
Admin:
  Checked that a Tungsten build still works on a build machine with no other
  perl installation. "perl" was aliased to an error to ensure it wasn't used.

Version 5.54. Not tagged

Makefile

@@ -64,10 +64,11 @@ MODSQZ  = modsqz
 OBJASM  = objasm
 RM      = remove
 WIPE    = -wipe
+PERL    = do <Perl$Dir>.perl
 
 # Note the output redirection on the end of this macro to enable parameters
 # to match those of the old application xtentries.
-XTENT   = perl build:xtentries >
+XTENT   = ${PERL} build:xtentries >
 
 DIFF    = gnu.diff -df >null: