Merge branch 'maint'

Merge branch 'maint'

* maint:
  tests: remove most uses of the AM_PROG_CC_C_O obsolete macro
  coverage: obsolete macro AM_PROG_CC_C_O should cause no warning nor errors
  INSTALL: update copyright years
  ithreads: use runtime (not configure time) detection of perl threads
  copyright: add few missing copyright notices
  maint: files in PLANS are to be exempted from copyright notice
  maint: consistently honor the UPDATE_COPYRIGHT_YEAR environment variable
  copyright: update some copyright years

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>