Fix some Disc build errors
Jeffrey Lee authored
Detail:
  Makefiles/AAsmModule - Reintroduce IfThere check of LocalRes:Messages, so that components which don't have any messages file will build correctly once again
  Makefiles/CApp - Ensure ${INSERTVERSION}'d files are writable; fixes errors when performing repeated Disc builds of components which mark their install directory tree as readonly.
Admin:
  Fixes issue reported on forums with DDEUtils not being present in recent disc images:
  https://www.riscosopen.org/forum/forums/11/topics/2848


Version 6.51. Tagged as 'BuildSys-6_51'
99d15696