To overcome this problem, delete directory .deps (or its content) in /root/WL700g/nasoc/src/apps/popt directory and run ./configure again.
Maybe its a good idea to delete all .deps in whole project ... I have tried it and it seems nothing was broken.
Another issue was unreferenced errno.h, fixed by some manual includes (my system is Debian 3.1)
#include <errno.h>
This is the last point of my compilation attempt:
[ ! -f raidtools/Makefile ] || make -C raidtools
make[2]: Entering directory `/root/WL700g/nasoc/src/apps/raidtools'
mipsel-linux-gcc -o raidstart raidstart.o parser.o raidlib.o version.o raid_io.o scsi.o reconfiguration.o rrc_common.o rrc_raid0.o rrc_raid5.o rrc_single.o ../popt/libpopt.a
raid_io.o: In function `raidseek':raid_io.c: (.text+0x3d4) : undefined reference to `_llseek'
:raid_io.c: (.text+0x3e8) : undefined reference to `_llseek'
collect2: ld returned 1 exit status
make[2]: *** [raidstart] Error 1
make[2]: Leaving directory `/root/WL700g/nasoc/src/apps/raidtools'
make[1]: *** [raidtools] Error 2
make[1]: Leaving directory `/root/WL700g/nasoc/src/apps'
make: *** [rebuild] Error 2
I am not able to resolve this error.