Merge branch 'ng/clean-many' into ng/master

* ng/clean-many:
  [ng] clean: do not exceed command line length limits, even with many files
  [ng] $(am__newline): will not be stripped unexpectedly
  [ng] coverage: $(am__newline) should never be stripped off
  [ng] am.xargs-map: be usable with huge lists
  [ng] am.xargs-map: new internal make function
  [ng] coverage: cleaning rules with lots of files to clean

diff --cc lib/am/header-vars.am
Simple merge