Merge branches 'ng/require-gmake-3.81' and 'ng/decruft' into ng/master

* ng/require-gmake-3.81:
  [ng] requirements: require GNU make >= 3.81

* ng/decruft:
  [ng] cleanup: don't support $(ACLOCAL_M4_SOURCES) anymore, it's obsolete
  [ng] compile: don't support $(INCLUDES) anymore, it's obsolete
  [ng] mkinstalldirs: remove support for the script and the variable
  [ng] cleanup: minor cleanups in remake-hdr.am
  [ng] config.h.{bot,top}: don't support anymore (distribution and deps)
  [ng] acconfig.h: don't support anymore (distribution and dependencies)
  [ng] cleanup: remove obsolete special-handling of default source for libs
  [ng] cleanup: remove a workaround for BSD make (default sources and VPATH)
  [ng] cleanup: remove a workaround for BSD make (comments continuation)
  [ng] cleanup: remove obsolescent comments about subdir-objects and deptrack
  [ng] cleanup: remove a workaround only needed by Solaris make
  [ng] push_dist_common: don't directly append to an Automake::Variable