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

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

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(+)

diff --git a/git-compat-util.h b/git-compat-util.h
index 1229c8296b9..69d9b5f202f 100644
--- a/git-compat-util.h
+++ b/git-compat-util.h
@@ -318,7 +318,25 @@ static inline int setitimer(int which, const struct itimerval *value, struct iti
 #endif
 
 #ifndef NO_LIBGEN_H
+/*
+ * FreeBSD's libgen.h inadvertently requires C11 features, due to its
+ * sys/cdefs.h using _Generic() if
+ * __has_extension(c_generic_selections) is true, regardless of
+ * __STDC_VERSION__....
+ */
+#if defined(__FreeBSD__) && __STDC_VERSION__ - 0 < 201112L
+#include <sys/cdefs.h>
+#ifdef __generic
+#define __fbsd_generic __generic
+#endif
+#undef __generic
+#endif
 #include <libgen.h>
+/* ...continue FreeBSD-specific hack above */
+#ifdef __fbsd_generic
+#define __generic __fbsd_generic
+#undef __fbsd_generic
+#endif
 #else
 #define basename gitbasename
 char *gitbasename(char *);
-- 
2.35.0.rc1.862.gadd77378860