Merge branch '2.6.3' into maint

Merge branch '2.6.3' into maint

* 2.6.3: (22 commits)
  maint: post-release administrivia
  version 2.6.3
  gnulib: update
  tests: check %no-lines
  NEWS: warnings with clang
  warnings: avoid warnings from clang
  tests: no longer disable -O compiler options
  yacc.c: initialize yylval in pure-parser mode
  skeletons: style changes
  tests: minor improvements
  tests: use $PERL instead of perl
  build: look for Perl in configure.
  tests: fix sed portability issues
  tests: diff -u is not portable
  maint: word changes
  lalr1.cc: fix test suite portability
  maint: fix an erroneous include
  tests: check that headers are self contained
  doc: add missing documentation for --report
  headers: move CPP guards into YY_*_INCLUDED to avoid collisions
  ...

Conflicts:
	NEWS
	doc/bison.texi
	tests/headers.at