improve strmake_buf() to detect wrong usage reliably

strmake_buf() macro should only be used with char[] arrays,
and never with char* pointers. To distinguish between the two
we create a new variable of the same type and initialize it
using array initializer, this causes compilation failure
with pointers. The variable is unused and will be removed by the
compiler. It's enough to do this check only with gcc, so
it doesn't have to be portable.

include/m_string.h

@@ -99,7 +99,7 @@ extern	char *strmake(char *dst,const char *src,size_t length);
 #define strmake_buf(D,S)        strmake(D, S, sizeof(D) - 1)
 #else
 #define strmake_buf(D,S) ({                             \
-  compile_time_assert(sizeof(D) != sizeof(char*));      \
+  typeof (D) __x __attribute__((unused)) = { 2 };       \
   strmake(D, S, sizeof(D) - 1);                         \
   })
 #endif