gcc10 now defaults to -fno-common (ref: https://gcc.gnu.org/gcc-10/changes.html ). Now with compiled with gcc10, multiple definitions are found which causes linker error as:
* --with-tcl:
```
/usr/bin/ld: table/table3D/cfunc.o:/builddir/build/BUILD/ngspice-32/ngspice/tclspice/src/xspice/icm/../../../src/include/../xspice/icm/dlmain.h:8: multiple definition of `coreitf'; table/dlmain.o:/builddir/build/BUILD/ngspice-32/ngspice/tclspice/src/xspice/icm/dlmain.c:45: first defined here
```
* --enable-oldapps
```
/bin/ld: frontend/com_history.o:(.data+0x4): multiple definition of `cp_maxhistlength'; ngsconvert.o:(.bss+0xa4): first defined here
collect2: error: ld returned 1 exit status
```
This commit fixes these issues.