Using an echo program instead of gcc, when running gprbuild

crud:64 trashman: gprbuild --compiler-subst=Ada,echo doors_optimized.adb                                                                                       (/tmp)
using project file /usr/share/gpr/_default.gpr
Compile
   [Ada]          doors_optimized.adb
-c -x ada -gnatA -gnatec=/tmp/GNAT-TEMP-000003.TMP -gnatem=/tmp/GNAT-TEMP-000004.TMP /tmp/doors_optimized.adb
Bind
   [gprbind]      doors_optimized.bexch
   [Ada]          doors_optimized.ali
gnatbind: Cannot find: doors_optimized.ali
gprbind: invocation of gnatbind failed
gprbuild: unable to bind doors_optimized.adb
crud:64 trashman: