Merge remote-tracking branch 'origin/maint'

* origin/maint:
  glr.cc: fix a clang warning
  maint: update copyright years
  build: fix VPATH issue
  build: avoid clang's colored diagnostics in the test suite
  tests: please clang and use ".cc", not ".c", for C++ input
  gnulib: update
  skeletons: avoid empty switch constructs
  lalr1.cc: fix compiler warnings
  yacc.c: do not use __attribute__ unprotected
  tests: style changes

tests/glr-regression.at

@@ -486,8 +486,8 @@ start:
    ;
 
 %%
-]AT_YYLEX_DEFINE(["a"])[
 ]AT_YYERROR_DEFINE[
+]AT_YYLEX_DEFINE(["a"])[
 ]AT_MAIN_DEFINE[
 ]])
 AT_BISON_OPTION_POPDEFS
@@ -1679,8 +1679,8 @@ start:
 b: 'b';
 d: /* nada.  */;
 %%
-]AT_YYLEX_DEFINE(["abc"])[
 ]AT_YYERROR_DEFINE[
+]AT_YYLEX_DEFINE(["abc"])[
 ]AT_MAIN_DEFINE[
 ]])
 AT_BISON_OPTION_POPDEFS