macros.h, cleanup SGN() macro

src/include/ngspice/macros.h

@@ -34,7 +34,7 @@
 #define MIN(a,b) ((a) < (b) ? (a) : (b))
 #define MAX(a,b) ((a) > (b) ? (a) : (b))
 #define ABS(a)    ((a) < 0.0 ? -(a) : (a))
-#define SGN(a)    ((a) < 0.0 ? -(1.0) : (1.0))
+#define SGN(a)    copysign(1.0, (a))
 #define SWAP(type, a, b)                        \
     do {                                        \
         type SWAP_macro_local = a;              \