Add VPATH support to Apache:

  os/$(OS_DIR) is now equivalent to the old $(OS_DIR)

  Necessary include paths are added only in VPATH mode.

  Dependencies are stored in the build directory now.
  They contain paths which depend on the current build,
  and therefore they are not general.

  Fixed two dependencies in main/Makefile.in.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@85113 13f79535-47bb-0310-9956-ffa450edef68