cad/ngspice_rework: fix build on GCC architectures

Use newer GCC:
dstring.o: In function `copy':
dstring.c:(.opd+0x240): multiple definition of `copy'
spice2poly/dlmain.o:dlmain.c:(.opd+0x6f0): first defined here

MFH: 2020Q3 (fix build blanket)