build: fix FreeBSD build when sysinfo compat library installed

build: fix FreeBSD build when sysinfo compat library installed

Commit 50dec7c566 ("config.mak.uname: add sysinfo() configuration for
cygwin", 2025-04-17) and later commit 187ce0222f ("configure.ac: upgrade
to a compilation check for sysinfo", 2025-05-19) added a 'sysinfo()'
check to the autoconf build.

The FreeBSD system has an optional sysinfo compatibility library, used
to assist in porting software, which causes the build to fail when it
is installed. The reason for the failure is the lack of '-lsysinfo'
during the linking step.

Several solutions were considered:

  - add a 'linking' check to configure.ac in order to determine the
    need to link a separate library (-lsysinfo). (This would require
    a similar change to meson.build).

  - change the order of the preprocessor conditionals in the total_ram()
    function in 'builtin/gc.c', so that the *BSD sysctl() function
    (in the HAVE_BSD_SYSCTL block) takes priority over the sysinfo()
    function (in the HAVE_SYSINFO block).

  - suppress the setting of HAVE_SYSINFO when HAVE_BSD_SYSCTL has been
    defined (in both configure.ac and meson.build).

The first solution above, while simple, adds unnecessary code (the
sysinfo compat function is likely implemented using sysctl() anyway)
when git is happy to use sysctl() on *BSD systems.

The second solution would only be required by the autoconf and meson
build systems, the Makefile already sets the build variables to the
required values (since they are not 'auto-detected').

Here we opt for the final solution above, since it only requires that
we prioritise the 'auto-detected' build variables in the autoconf and
meson builds.

In order to fix the FreeBSD build, move the sysinfo() check after the
determination of the HAVE_BSD_SYSCTL build variable, suppressing the
setting of HAVE_SYSINFO if HAVE_BSD_SYSCTL is defined. Apply this logic
to both the configure.ac and meson.build file.

[Thanks go to Renato Botelho <garga@FreeBSD.org> for testing this patch
on FreeBSD.]

Tested-by: Renato Botelho <garga@FreeBSD.org>
Signed-off-by: Ramsay Jones <ramsay@ramsayjones.plus.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>