tests: don't fail if the C++ compiler does not work

Also, make sure that `make dist` generates a correct tarball even if
the C++ compiler does not work.

Reported by Nelson H. F. Beebe.

* m4/cxx.m4 (BISON_CXX_WORKS): Define to true/false instead of
true/exit 77.  The latter is too dangerous to use (it directly quits).
(ENABLE_CXX): New name for the Automake conditional, for consistency
with ENABLE_CXX11 etc.
* tests/local.at (AT_COMPILE, AT_COMPILE_CXX): Adjust to the new
semantics of BISON_CXX_WORKS.
* examples/c++/local.mk: Skip the variant test if C++ does not work.
* examples/calc++/local.mk: Likewise.

examples/c++/local.mk

@@ -35,21 +35,25 @@ if ENABLE_CXX14
   %C%_simple_CXXFLAGS = $(CXX11_CXXFLAGS)
   # Don't use gnulib's system headers.
   %C%_simple_CPPFLAGS = -I$(top_builddir)
-  dist_TESTS += %D%/simple.test
+  TESTS += %D%/simple.test
   %D%/simple.cc: $(BISON_IN) $(dist_pkgdata_DATA)
 endif
+EXTRA_DIST += %D%/simple.test
 
 
 ## ---------- ##
 ## Variants.  ##
 ## ---------- ##
 
-check_PROGRAMS += %D%/variant
-nodist_%C%_variant_SOURCES = %D%/variant.yy
-# Don't use gnulib's system headers.
-%C%_variant_CPPFLAGS = -I$(top_builddir)
-dist_TESTS += %D%/variant.test
-%D%/variant.cc: $(BISON_IN) $(dist_pkgdata_DATA)
+if ENABLE_CXX
+  check_PROGRAMS += %D%/variant
+  nodist_%C%_variant_SOURCES = %D%/variant.yy
+  # Don't use gnulib's system headers.
+  %C%_variant_CPPFLAGS = -I$(top_builddir)
+  TESTS += %D%/variant.test
+  %D%/variant.cc: $(BISON_IN) $(dist_pkgdata_DATA)
+endif
+EXTRA_DIST += %D%/variant.test
 
 if ENABLE_CXX11
   check_PROGRAMS += %D%/variant-11
@@ -57,9 +61,10 @@ if ENABLE_CXX11
   %C%_variant_11_CXXFLAGS = $(CXX11_CXXFLAGS)
   # Don't use gnulib's system headers.
   %C%_variant_11_CPPFLAGS = -I$(top_builddir)
-  dist_TESTS += %D%/variant-11.test
+  TESTS += %D%/variant-11.test
   %D%/variant-11.cc: $(BISON_IN) $(dist_pkgdata_DATA)
 endif
+EXTRA_DIST += %D%/variant-11.test
 
 dist_cxx_DATA = %D%/README %D%/Makefile %D%/variant.yy %D%/variant-11.yy
 CLEANFILES += %D%/simple.output %D%/variant.output %D%/variant-11.output

examples/calc++/local.mk

@@ -65,17 +65,16 @@ calcxx_sources =                                \
   $(calcxx_sources_generated)
 
 if FLEX_CXX_WORKS
-check_PROGRAMS += %D%/calc++
-nodist_%C%_calc___SOURCES =                     \
-  $(calcxx_sources)
-
-# Don't use gnulib's system headers.
-%C%_calc___CPPFLAGS = -I$(top_builddir)/%D%
-%C%_calc___CXXFLAGS = $(AM_CXXFLAGS) $(FLEX_SCANNER_CXXFLAGS)
-dist_TESTS += %D%/calc++.test
-else
+if  ENABLE_CXX
+  check_PROGRAMS += %D%/calc++
+  nodist_%C%_calc___SOURCES = $(calcxx_sources)
+  # Don't use gnulib's system headers.
+  %C%_calc___CPPFLAGS = -I$(top_builddir)/%D%
+  %C%_calc___CXXFLAGS = $(AM_CXXFLAGS) $(FLEX_SCANNER_CXXFLAGS)
+  TESTS += %D%/calc++.test
+endif  ENABLE_CXX
+endif FLEX_CXX_WORKS
 EXTRA_DIST += %D%/calc++.test
-endif
 
 
 ## ------------ ##

m4/cxx.m4

@@ -50,14 +50,12 @@ AC_DEFUN([BISON_TEST_FOR_WORKING_CXX_COMPILER],
     AC_LANG_POP([C++])])
 
  case $bison_cv_cxx_works in
- yes)
-    BISON_CXX_WORKS=':';;
- no | cross)
-    BISON_CXX_WORKS='exit 77';;
+   yes)        BISON_CXX_WORKS=':';;
+   no | cross) BISON_CXX_WORKS='false';;
  esac
 
  AC_SUBST([BISON_CXX_WORKS])
- AM_CONDITIONAL(BISON_CXX_WORKS, test $bison_cv_cxx_works = yes)
+ AM_CONDITIONAL([ENABLE_CXX], [test $bison_cv_cxx_works = yes])
 ])
 
 # BISON_CXX_COMPILER_POSIXLY_CORRECT

tests/atlocal.in

@@ -98,10 +98,8 @@ fi
 : ${CXX_COMPILER_POSIXLY_CORRECT='@CXX_COMPILER_POSIXLY_CORRECT@'}
 
 if $POSIXLY_CORRECT_IS_EXPORTED; then
-  $C_COMPILER_POSIXLY_CORRECT ||
-    BISON_C_WORKS="as_fn_error 77 POSIXLY_CORRECT"
-  $CXX_COMPILER_POSIXLY_CORRECT ||
-    BISON_CXX_WORKS="as_fn_error 77 POSIXLY_CORRECT"
+  $C_COMPILER_POSIXLY_CORRECT || BISON_C_WORKS=false
+  $CXX_COMPILER_POSIXLY_CORRECT || BISON_CXX_WORKS=false
 fi
 
 # Handle --compile-c-with-cxx here, once CXX and CXXFLAGS are known.

tests/local.at

@@ -777,7 +777,7 @@ m4_define([AT_QUELL_VALGRIND],
 # otherwise pass "-c"; this is a hack.  The default SOURCES is OUTPUT
 # with trailing .o removed, and ".c" appended.
 m4_define([AT_COMPILE],
-[AT_CHECK([$BISON_C_WORKS], 0, ignore, ignore)
+[AT_SKIP_IF([[! $BISON_C_WORKS]])
 AT_CHECK(m4_join([ ],
                  [$CC $CFLAGS $CPPFLAGS $3],
                  [m4_bmatch([$1], [[.]], [-c], [$LDFLAGS])],
@@ -797,7 +797,7 @@ AT_CHECK(m4_join([ ],
 # with trailing ".o" removed, and ".cc" appended.
 m4_define([AT_COMPILE_CXX],
 [AT_KEYWORDS(c++)
-AT_CHECK([$BISON_CXX_WORKS], 0, ignore, ignore)
+AT_SKIP_IF([[! $BISON_CXX_WORKS]])
 AT_CHECK(m4_join([ ],
                  [$CXX $CXXFLAGS $CPPFLAGS $3],
                  [m4_bmatch([$1], [[.]], [-c], [$LDFLAGS])],