Skip sys/sysctl.h on linux in mono

[am11]

Fixes three warnings:

  [ 56%] Building C object mono/mini/CMakeFiles/monosgen-objects.dir/__/utils/mono-mmap.c.o
  In file included from /runtime/src/mono/mono/utils/mono-mmap.c:23:
  /usr/include/x86_64-linux-gnu/sys/sysctl.h:21:2: warning: #warning "The <sys/sysctl.h> header is deprecated and will be removed." [-Wcpp]
     21 | #warning "The <sys/sysctl.h> header is deprecated and will be removed."
        |  ^~~~~~~
# snip
  [ 59%] Building C object mono/mini/CMakeFiles/monosgen-objects.dir/__/utils/mono-time.c.o
  In file included from /runtime/src/mono/mono/utils/mono-proclib.c:42:
  /usr/include/x86_64-linux-gnu/sys/sysctl.h:21:2: warning: #warning "The <sys/sysctl.h> header is deprecated and will be removed." [-Wcpp]
     21 | #warning "The <sys/sysctl.h> header is deprecated and will be removed."
        |  ^~~~~~~
  [ 59%] Building C object mono/mini/CMakeFiles/monosgen-objects.dir/__/utils/mono-uri.c.o
  [ 59%] Building C object mono/mini/CMakeFiles/monosgen-objects.dir/__/utils/mono-poll.c.o
  In file included from /runtime/src/mono/mono/utils/mono-time.c:107:
  /usr/include/x86_64-linux-gnu/sys/sysctl.h:21:2: warning: #warning "The <sys/sysctl.h> header is deprecated and will be removed." [-Wcpp]
     21 | #warning "The <sys/sysctl.h> header is deprecated and will be removed."
        |  ^~~~~~~

With this PR: 0 warnings - http://sprunge.us/96kyq9

Like in src/libraries/Native, skip linux from sys/sysctl.h check. For other systems, check_include_files (instead of check_include_file (singular)) when a header depends on another one.

Also, simplified few conditions: "is linux" check supersedes "is android".

[am11]

cc @akoeplinger

[Thefrank]

FYI: FreeBSD doesn't have a mono section in CI yet as #57487 is still pending.

I don't foresee the change breaking FreeBSD

[am11]

The introspection check for sysctl.h is ported from libs.native subset, so it is as good / safe. Harmonizing configs generally reduces the risk of breakage. :)

[am11]

It would be nice to replace ac_check_headers function with explicit check_include_files() calls (one line per header), like we have in coreclr, libs, corehost and tests. Some of the headers are unused, and some have dependencies (like sys/sysctl.h requires sys/types.h).
@akoeplinger, @vargaz, @lambdageek, wdyt? I can give it a go as part of this PR.

[akoeplinger]

I can give it a go as part of this PR.

I prefer to do it in a separate PR. We could also add parameter(s) for the dependent headers to ac_check_headers to keep the shared HAVE_* logic, but I don't feel strongly about it.