devel/libgit2: fix build with clang 16

Clang 16 has a new error about incompatible function types, which shows
up when building devel/libgit2:

/wrkdirs/usr/ports/devel/libgit2/work/libgit2-1.5.2/src/util/util.c:731:28: error: incompatible function pointer types passing 'int (void *, const void *, const void *)' to parameter of type 'int (*)(const void *, const void *, void *)' [-Wincompatible-function-pointer-types]
        qsort_s(els, nel, elsize, git__qsort_r_glue_cmp, &glue);
                                  ^~~~~~~~~~~~~~~~~~~~~
/usr/include/stdlib.h:397:11: note: passing argument to parameter here
    int (*)(const void *, const void *, void *), void *);
          ^

Clang is indeed right, as the version of qsort_s(3) in FreeBSD 13 and
later has the 'void *payload' parameter last:

errno_t  qsort_s(void *, rsize_t, rsize_t,
    int (*)(const void *, const void *, void *), void *);

This could be fixed by putting the arguments in the right place for
qsort_s(3), but it turns out the rabbit hole goes a bit deeper: on
14-CURRENT, libgit2's CMake configuration is not able to detect
qsort_r(3), which is actually why it chooses qsort_s():

• Checking prototype qsort_r for GIT_QSORT_R_BSD
• Checking prototype qsort_r for GIT_QSORT_R_BSD - False
• Checking prototype qsort_r for GIT_QSORT_R_GNU
• Checking prototype qsort_r for GIT_QSORT_R_GNU - False
• Looking for qsort_s
• Looking for qsort_s - found

The problem with the GIT_QSORT_R_BSD detection is due to the check in
libgit2's src/CMakeLists.txt, where it does:

check_prototype_definition(qsort_r
        "void qsort_r(void *base, size_t nmemb, size_t size, void
        *thunk, int (*compar)(void *, const void *, const void *))"
        "" "stdlib.h" GIT_QSORT_R_BSD)

and CMake attempts to define a function with a similar prototype in its
test program, which then fails to compile, at least on 14-CURRENT:

/wrkdirs/share/dim/ports/devel/libgit2/work/.build/CMakeFiles/CMakeScratch/TryCompile-tILE28/CheckPrototypeDefinition.c:14:6:
error: expected identifier or '('
void qsort_r(void *base, size_t nmemb, size_t size, void *thunk, int
(*compar)(void *, const void *, const void *)) {
     ^
/usr/include/stdlib.h:357:5: note: expanded from macro 'qsort_r'
    __generic(arg5, int (*)(void *, const void *, const void *),
     \\
    ^
/usr/include/sys/cdefs.h:323:2: note: expanded from macro '__generic'
        _Generic(expr, t: yes, default: no)
        ^

This is because in https://cgit.freebsd.org/src/commit/?id=af3c78886fd8d
Ed Schouten changed the prototype of qsort_r(3) to match POSIX, using a
C11 _Generic macro. When CMake tries to compile its own custom
definition of qsort_r, that fails with the above compile error, because
the macro gets expanded in place of the function declaration.

So the summarized situation is:

• On 12.x and 13.x, qsort_r(3) is a plain function, and uses the old comparison function type: 'int (*compar)(void *thunk, const void *elem1, const void *elem2)'. Therefore, CMake detects GIT_QSORT_R_BSD, and libgit2's src/util/util.c uses the correct comparison function type.
• On 14.x, qsort_r(3) is a macro, and uses the POSIX comparison function type: 'int (*compar)(const void *elem1, const void *elem2, void *thunk)'. Therefore, CMake fails to detect GIT_QSORT_R_BSD, and detects GIT_QSORT_S instead, and libgit2's src/util/util.c uses an incorrect comparison function type.

I submitted https://github.com/libgit2/libgit2/pull/6555 and
https://github.com/libgit2/libgit2/pull/6558 upstream to remedy the
situation and correctly detect all qsort variants, and these were merged
with a few minor changes.

This is an adjusted version of the upstream commits that applies on top
of libgit-1.5.2.

PR: 271234
Approved by: mfechner (maintainer)
MFH: 2023Q2