gnuplot.c: add missing header; do not link to unavailable libquadmath on PPC; makefiles: consistently use gfortran

[barracuda156]

Re gnuplot.c:

src/gnuplot.c: In function 'gnuplot_create':
src/gnuplot.c:215:13: warning: implicit declaration of function 'strcmp' [-Wimplicit-function-declaration]
  215 |         if(!strcmp(argv[1], "--noplot"))
      |             ^~~~~~
src/gnuplot.c:23:1: note: include '<string.h>' or provide a declaration of 'strcmp'
   22 | #include "dspl.h"
  +++ |+#include <string.h>
   23 | 
src/gnuplot.c:237:13: warning: implicit declaration of function 'memset' [-Wimplicit-function-declaration]
  237 |             memset(str, 0, 1024*sizeof(char));
      |             ^~~~~~
src/gnuplot.c:237:13: note: include '<string.h>' or provide a declaration of 'memset'
src/gnuplot.c:237:13: warning: incompatible implicit declaration of built-in function 'memset' [-Wbuiltin-declaration-mismatch]
src/gnuplot.c:237:13: note: include '<string.h>' or provide a declaration of 'memset'

The rest should be self-explanatory.

[barracuda156]

@Dsplib If there is something wrong, we could fix it rather than closing and leaving issues unfixed.