Fix compiler warnings caused by -Wpointer-arith

src/Makefile

@@ -107,6 +107,7 @@ CFLAGS                  += -Wundef
 CFLAGS                  += -Wshadow
 CFLAGS                  += -Wmissing-declarations
 CFLAGS                  += -Wmissing-prototypes
+CFLAGS                  += -Wpointer-arith
 CFLAGS                  += -ftrapv
 
 LFLAGS                  :=

src/potfile.c

@@ -127,7 +127,7 @@ void *hc_bsearch_r (const void *key, const void *base, size_t nmemb, size_t size
 
     const size_t c = l + m;
 
-    const void *next = base + (c * size);
+    const char *next = (char *) base + (c * size);
 
     const int cmp = (*compar) (key, next, arg);