Support silent rules for PCH generation.

When silent rules are enabled (e.g. "make V=0"), generating the
StdAfx.gch file was hidden.  Now it displays along with the other rules,
example:

Making all in MazeCompiler
  GEN    StdAfx.h.gch
  CXX    hr_mazecompiler-main.o
  CXXLD  hr-mazecompiler
Generate dependency info for StdAfx.h.