tools/gnulib: allow forced exclusion of functions after configure

tools/gnulib: allow forced exclusion of functions after configure

Overriding variables used in both the macros and the headers
like setting REPLACE_FCNTL to 0 while invoking Make causes the
function aliases like rpl_fcntl() to not be defined,
however the object may still be built with the fnctl() function.

Usually this is enough for building while avoiding
the need to link the resulting libgnu library
to every single other build target for a project
in order to include the gnulib copy of the function,
because in these cases we don't care which version
of the function is used.

However for functions like fcntl() this doesn't work
as it is designed to use either the alias or standard declaration
from gnulib headers in order to be
a wrapper for the native host copy of fcntl()
by containing recursive calls to fcntl() within itself
after undefining the gnulib function declaration.

Overriding the variables used by the header when invoking Make
causes the header's declarations to be blocked,
and this results in the gnulib version of fcntl()
to call itself recursively and indefinitely
leading to segmentation faults.

Fix this by using macros defined with those variables
in order to exclude the function during preprocessing.

While at it, do the same for reallocarray()
so that the configure option --avoid=reallocarray
and Make variable REPLACE_REALLOCARRAY set to 0
would have a similar effect.

In the future this patch can be expanded to include
more functions and some version of this may be upstreamable.

Signed-off-by: Michael Pratt <mcpratt@pm.me>
Link: https://github.com/openwrt/openwrt/pull/15690
Signed-off-by: Robert Marko <robimarko@gmail.com>