The message on configure is misleading:
checking whether the linker accepts -std=c++11... yes
checking whether the linker accepts -std=c++14... yes
checking whether the linker accepts -std=c++17... no
It is the compiler that we check, not just the linker.
* m4/ax_check_link_flag.m4: Remove.
* m4/bison-check-compiler-flag.m4: New.
* m4/bison-cxx-std.m4: Use it.
On the CI, we had failures such as:
./c++.at:401: $PREPARSER ./list
stderr:
./list: error while loading shared libraries: libc++.so.1:
cannot open shared object file: No such file or directory
because we accepted `-std=c++ -stdlib=libc++` although libc++ is not
installed on the machine.
* m4/ax_check_compile_flag.m4 (AX_CHECK_COMPILE_FLAG): Rewrite as...
* m4/bison-check-compile-flag.m4 (BISON_CHECK_COMPILE_FLAG): this, so
that we use AC_LINK_IFELSE to check the compiler (and its std lib)
instead of AC_COMPILE_IFELSE.
On the CI, we have this spurious failure with clang 3.9 with
-std=c++17:
In file included from list.y:23:
In file included from /usr/include/c++/4.8/iostream:39:
In file included from /usr/include/c++/4.8/ostream:38:
In file included from /usr/include/c++/4.8/ios:42:
In file included from /usr/include/c++/4.8/bits/ios_base.h:41:
In file included from /usr/include/c++/4.8/bits/locale_classes.h:40:
In file included from /usr/include/c++/4.8/string:52:
In file included from /usr/include/c++/4.8/bits/basic_string.h:2815:
In file included from /usr/include/c++/4.8/ext/string_conversions.h:43:
/usr/include/c++/4.8/cstdio:120:11: error: no member named 'gets' in the global namespace
using ::gets;
~~^
This shows that our test, based on gl_WARN_ADD, is a joke. We have to
really check for at least a bit of C++.
* m4/ax_check_compile_flag.m4, m4/bison-cxx-std.m4: New.
* configure.ac: Use them to make sure the compiler actually works.
