Make use of GCC's __attribute__((unused)) (#6392)

* configure.ac: Add support for GCC's __attribute__((unused))

When compiling under GCC with -Wextra (along with -Wall) there are a lot
of compiler warnings such as

collectors/cgroups.plugin/cgroup-network.c:89:45: warning: unused parameter ‘scope’ [-Wunused-parameter]
   89 | struct iface *read_proc_net_dev(const char *scope, const char *prefix) {
      |                                 ~~~~~~~~~~~~^~~~~

Some arguments may be able to be got rid off, others won't.

GCC (and at least clang[0]) provide an __attribute__((unused))
annotation that can be used on function parameters (also on variables,
functions, labels, enums, structs etc) to inform the compiler of such
and will squash warnings of the above nature.

A check is added to configure.ac for the use of GCC (I believe $GCC will
be set to 'yes' for clang also) and if found it creates __always_unused
& __maybe_unused #define's set to __attribute__((unused)) otherwise it
just sets them empty.

If other compilers have a similar feature this check can be adjusted to
accommodate them.

The reason for the two defines is that some variables may always be
unused in a function, others may or may not depending on #ifdef's for
example. So we are able to document both cases.

Subsequent commits will start making use of these to squash such
compiler warnings.

[0]: https://clang.llvm.org/docs/AttributeReference.html#maybe-unused-unused

Signed-off-by: Andrew Clayton <andrew@zeta.digital-domain.net>

* collectors/statsd.plugin: Mark a function argument as __maybe_unused

In collectors/statsd.plugin/statsd.c the app function argument to
STATSD_APP_CHART_DIM() might be unused if NETDATA_INTERNAL_CHECKS is not
defined, then the debug() macro that it's used in from
libnetdata/log/log.h will be defined to a dummy function where none of
the arguments are used.

This fixes a compiler warning [-Wunused-parameter] when compiling with
-Wextra *and* -Wall.

Signed-off-by: Andrew Clayton <andrew@zeta.digital-domain.net>

* collectors/apps.plugin: Mark a function argument as __maybe_unused

In collectors/apps.plugin/apps_plugin.c the function
debug_print_process_tree() takes an argument 'msg' that might be unused
if NETDATA_INTERNAL_CHECKS is not defined, then debug_log() will be set
to a dummy function that takes no arguments.

This fixes a compiler warning [-Wunused-parameter] when compiling with
-Wextra *and* -Wall.

Signed-off-by: Andrew Clayton <andrew@zeta.digital-domain.net>

* libnetdata/locks/locks: Mark function arguments as __maybe_unused

In libnetdata/locks/locks.c there a number of functions that take
arguments 'file', 'function' & 'line' that might be unused if
NETDATA_INTERNAL_CHECKS is not defined, then the debug() macro that it's
used in from libnetdata/log/log.h will be defined to a dummy function
where none of the arguments are used.

This fixes compiler warnings [-Wunused-parameter] when compiling with
-Wextra *and* -Wall.

Signed-off-by: Andrew Clayton <andrew@zeta.digital-domain.net>

* libnetdata/socket/security: Mark a function argument as __maybe_unused

In libnetdata/socket/security.c the function security_info_callback()
takes an argument 'ret' that might be unused if NETDATA_INTERNAL_CHECKS
is not defined, then the debug() macro that it's used in from
libnetdata/log/log.h will be defined to a dummy function where none of
the arguments are used.

This fixes a compiler warning [-Wunused-parameter] when compiling with
-Wextra *and* -Wall.

Signed-off-by: Andrew Clayton <andrew@zeta.digital-domain.net>

* collectors/cgroups.plugin: Mark a function argument as __maybe_unused

In collectors/cgroups.plugin/cgroup-network.c the function
read_proc_net_dev() takes an argument 'scope' that might be unused if
the NETDATA_INTERNAL_CHECKS is not defined.

This fixes a compiler warning [-Wunused-parameter] when compiling with
-Wextra *and* -Wall".

Signed-off-by: Andrew Clayton <andrew@zeta.digital-domain.net>

* database/rrddim: Mark function arguments as __maybe_unused

In database/rrddim.c there a couple of functions that take a 'st'
argument that might be unused if NETDATA_INTERNAL_CHECKS is not defined,
then the debug() macro that it's used in from libnetdata/log/log.h will
be defined to a dummy function where none of the arguments are used.

This fixes compiler warnings [-Wunused-parameter] when compiling with
-Wextra *and* -Wall.

Signed-off-by: Andrew Clayton <andrew@zeta.digital-domain.net>

* database/rrdvar: Mark a function argument as __maybe_unused

In database/rrdvar.c the function rrdvar_create_and_index() take an
argument 'scope' that might be unused if NETDATA_INTERNAL_CHECKS is not
defined, then the debug() macro that it's used in from
libnetdata/log/log.h will be defined to a dummy function where none of
the arguments are used.

This fixes a compiler warning [-Wunused-parameter] when compiling with
-Wextra *and* -Wall.

Signed-off-by: Andrew Clayton <andrew@zeta.digital-domain.net>

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