[PATCH] config.mak.dev: fix DEVELOPER=1 on FreeBSD with -std=gnu99

Since the addition of -std=gnu99 to CFLAGS under DEVELOPER=1 in 5f46385309b (config.mak.dev: specify -std=gnu99 for gcc/clang, 2021-12-08) the compilation has been broken on newer FreeBSD systems. E.g. on FreeBSD 13 we'll now get this when compiling archive.o:

    archive.c:337:35: error: '_Generic' is a C11 extension [-Werror,-Wc11-extensions]
                    strbuf_addstr(&path_in_archive, basename(path));
                                                    ^
    /usr/include/libgen.h:61:21: note: expanded from macro 'basename'
    #define basename(x)     __generic(x, const char *, __old_basename, basename)(x)
                            ^
    /usr/include/sys/cdefs.h:325:2: note: expanded from macro '__generic'
            _Generic(expr, t: yes, default: no)
            ^

This issue is a bug in FreeBSD introduced in freebsd/freebsd-src@62b7f85d4749 (Leave the C11 keywords alone when we have a recent version of GCC., 2014-09-03)[1].

That commit started conditionally using the C11 _Generic() keyword as a function of either a __STDC_VERSION__ check, or __has_extension(c_generic_selections). The current FreeBSD code in sys/cdefs.h being (slightly modified since that commit):

    #if (defined(__STDC_VERSION__) && __STDC_VERSION__ >= 201112L) || \
        __has_extension(c_generic_selections)
    [...]

The inherent problem is that __has_extension() is a way to ask Clang (and GCC) whether the compiler supports that feature, but the -Wc11-extensions warning will be issued on the basis of the selected __STDC_VERSION__. With -std=gnu99 the __has_extension() built-in will return true, but the warning will still fire.

Let's narrowly work around this by checking whether:

 A. We are under FreeBSD
 B. We are using a C standard older than C11

In that case we'll include sys/cdefs.h, and undefine FreeBSD's __generic() wrapper if it's been defined. We'll then load libgen.h, and restore whatever __generic definition we temporarily undefined earlier.

An alternate solution would be to simply define NO_LIBGEN_H=Y in config.mak.uname for FreeBSD, but this way we'll use its OS-provided basename() and dirname(), and in the general case ensure that nothing changes outside of DEVELOPER=1 builds.

1. https://github.com/freebsd/freebsd-src/commit/62b7f85d4749

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---
 git-compat-util.h | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)