Merge branch 'jn/makefile-cleanup'

Merge branch 'jn/makefile-cleanup'

Tightens dependency rules to avoid unnecessary recompilation, and
cleans up our Makefile in general.

* jn/makefile-cleanup:
  Makefile: document ground rules for target-specific dependencies
  Makefile: move GIT-VERSION-FILE dependencies closer to use
  Makefile: build instaweb similar to other scripts
  Makefile: update scripts when build-time parameters change
  Makefile: do not replace @@GIT_VERSION@@ in shell scripts
  Makefile: split prefix flags from GIT-CFLAGS
  Makefile: be silent when only GIT_USER_AGENT changes
  Makefile: split GIT_USER_AGENT from GIT-CFLAGS
  Makefile: do not replace @@GIT_USER_AGENT@@ in scripts
  Makefile: apply dependencies consistently to sparse/asm targets
  Makefile: do not have git.o depend on common-cmds.h
  Makefile: fold XDIFF_H and VCSSVN_H into LIB_H
  Makefile: fold MISC_H into LIB_H
  Makefile: sort LIB_H list