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.
cf1446dc9e