[PATCH 0/3] fix minor Makefile annoyances

I've noticed that running "make" on a fully-built git tree still builds a lot of things:

  $ make >/dev/null 2>&1 && make
      GEN perl/PM.stamp
      SUBDIR gitweb
      GEN git-add--interactive
      GEN git-difftool
      GEN git-archimport
      GEN git-cvsexportcommit
      GEN git-cvsimport
      GEN git-cvsserver
      GEN git-relink
      GEN git-send-email
      GEN git-svn
      GEN git-p4
      SUBDIR ../
  make[2]: 'GIT-VERSION-FILE' is up to date.
      GEN git-instaweb
      SUBDIR perl
      SUBDIR templates

Here it is after this series:

  $ make >/dev/null 2>&1 && make
      SUBDIR perl
      SUBDIR templates

The patches are:

  [1/3]: Makefile: drop dependency between git-instaweb and gitweb
  [2/3]: Makefile: avoid timestamp updates to GIT-BUILD-OPTIONS
  [3/3]: Makefile: silence perl/PM.stamp recipe

-Peff