The problem is that, in the patch phase, the patch file is referred to
as 'patch', but the package for the `patch` program is what that
variable resolves to:

(let ((patch.gz (assoc-ref inputs "patch")))

After fixing that by renaming the variable for the patch file to
something unique, the build fails like this:

------
gcc -D_GNU_SOURCE -O2 -Wall -g  -I. -idirafter ./include/ -Ilib   -c -o ipmaddr.o ipmaddr.c
ipmaddr.c: In function ‘multiaddr_modify’:
ipmaddr.c:351:25: warning: pointer targets in passing argument 2 of ‘parse_lla’ differ in signedness [-Wpointer-sign]
    if (parse_lla(*argv, ifr.ifr_hwaddr.sa_data) < 0)
                         ^
ipmaddr.c:78:12: note: expected ‘unsigned char *’ but argument is of type ‘char *’
 static int parse_lla(char *str, unsigned char *addr)
            ^
gcc  -Llib -o ipmaddr ipmaddr.o -lnet-tools 
gcc -D_GNU_SOURCE -O2 -Wall -g  -I. -idirafter ./include/ -Ilib   -c -o mii-tool.o mii-tool.c
mii-tool.c: In function ‘main’:
mii-tool.c:432:29: error: too few arguments to function ‘parse_media’
  case 'A': nway_advertise = parse_media(optarg); break;
                             ^
mii-tool.c:163:12: note: declared here
 static int parse_media(char *arg, unsigned *bmcr2)
            ^
mii-tool.c:433:26: error: too few arguments to function ‘parse_media’
  case 'F': fixed_speed = parse_media(optarg); break;
                          ^
mii-tool.c:163:12: note: declared here
 static int parse_media(char *arg, unsigned *bmcr2)
            ^
mii-tool.c:456:2: warning: implicit declaration of function ‘version’ [-Wimplicit-function-declaration]
  version();
  ^
mii-tool.c: At top level:
mii-tool.c:32:13: warning: ‘Version’ defined but not used [-Wunused-variable]
 static char Version[] = "$Id: mii-tool.c,v 1.9 2006/09/27 20:59:18 ecki Exp $\n(Author: David Hinds based on Donald Becker's mii-diag)";
             ^
make: *** [<builtin>: mii-tool.o] Error 1
phase `build' failed after 7.5 seconds
------

This problem with the wrong number of arguments being passed to
parse_media() is introduced in the Debian patch, so we should check if
there is a better version of the patch or if fixes are being applied
upstream.