Merge branch 'next'

* next: (21 commits)
  fix, PEBKAC, list errno-lit.def in CLEANFILES
  fix, PEBKAC, list strerrno.gperf.in as strerror SOURCE
  fix, ease certain compilers' (gcc, clang) minds and assure them the input is C indeed
  chore, list gperf as requisite for strerror in travis ci file
  minor, provide strerrno functionality, obtained from errno-lit.gperf and another awk script
  cosmetics, use (awk) sub instead of gsub
  minor, support errno-literal inputs (EINVAL, etc.)
  cosmetics, kick double inclusion of strerror-clo.h
  chore, always have gperf generate ansi C code
  minor, provide rules to generator a literal errno gperf file
  chore, also check for gperf tool
  fix, PEBKAC, check gengetopt presence through GENGETOPT shell var
  chore, check for both -dM and -fdirectives-only, and decide later which one to use
  minor, support icc's preprocessor (via -dM)
  fix, PEBKAC, check for sxe_check_flag in SXE_CHECK_PREPROC_FLAG
  minor, error out if preprocessor doesn't support -fdirectives-only
  chore, simplify SXE_CHECK_{COMPILER,PREPROC}_FLAG by always going for the non-literal case
  chore, clean up flags checking in sxe-compiler
  minor, check if preprocessor can do -fdirectives-only
  minor, provide more generic SXE_CHECK_CPP_FLAGS (modelled after SXE_CHECK_COMPILER_FLAGS)
  ...