Error making mkdep on Ultrix 4.5

I am trying to port Amaya 1.0b to a DECStation running Ultrix 4.3.
There is a problem with tools/mkdep: it uses strdup even though 
configure has worked out it is not present.

Here is what happened:

# ../configure
creating cache ./config.cache
checking host system type... mips-dec-ultrix4.5
checking target system type... mips-dec-ultrix4.5
checking build system type... mips-dec-ultrix4.5
  [skipping...]
checking for strdup... no
  [skipping...] 
Amaya and Thot packages are now configured
Amaya configured with Java support
Now start "make all" (GNU make preferably) to build the binaries
# make all
make[1]: Entering directory `/usr/local/Thot/objects/tools/mkdep'
gcc  -o mkdep mkdep.o
collect2: ld returned 1 exit status
/usr/lib/cmplrs/cc/ld:
Undefined:
strdup
make[1]: *** [mkdep] Error 1
make[1]: Leaving directory `/usr/local/Thot/objects/tools/mkdep'
make: *** [tools/mkdep/mkdep] Error 2
#


Peter Pudney