Remove dependency on libm

Just to save a few bytes when compiling for embedded systems.

Author: Marcel Meulemans

json-builder.c

@@ -33,12 +33,15 @@
 #include <string.h>
 #include <assert.h>
 #include <stdio.h>
-#include <math.h>
 
 #ifdef _MSC_VER
     #define snprintf _snprintf
 #endif
 
+static double floor(double d) {
+       return d - ((int)d % 1);
+}
+
 static const json_serialize_opts default_opts =
 {
    json_serialize_mode_single_line,