findutils: fix build w/glibc-2.34

This "-D__nonnull\\(params\\)=" leads to a compilation failure in e.g.
the configure phase:

    configure:21131: gcc -c -D__nonnull\(params\)=  conftest.c >&5
    <command-line>: warning: ISO C99 requires whitespace after the macro name
    <command-line>: error: stray '\' in program
    <command-line>: error: expected ',' or ';' before '(' token
    <command-line>: error: stray '\' in program

According to the commit this isn't even needed on Linux.

I confirmed that this is an (expectable) glibc-2.34 thing by checking
that

* the issue doesn't occur with gcc 10/11 on a recent glibc-2.33 staging.
* the issue DOES occur in a docker container with Fedora rawhide (which
  has glibc 2.34 and gcc 11).

pkgs/tools/misc/findutils/default.nix

@@ -40,7 +40,7 @@ stdenv.mkDerivation rec {
     "--localstatedir=/var/cache"
   ];
 
-  CFLAGS = [
+  CFLAGS = lib.optionals stdenv.isDarwin [
     # TODO: Revisit upstream issue https://savannah.gnu.org/bugs/?59972
     # https://github.com/Homebrew/homebrew-core/pull/69761#issuecomment-770268478
     "-D__nonnull\\(params\\)="