MAKE: ask gnulikes to warn and be pedantic + fixes

It seems clang 3.4 thinks the codebase is in fantastic shape and gcc 4.9.0
has only minor niggles, which I fixed:

- fix uninitialized member warning:
    In DEBUG mode the expr member doesn't get properly initialized to NULL.

- fix warnings about directive inside of macro's:
    On some platforms/compilers, sprintf is a macro. Putting macro directives
    inside of a macro is unportable and gcc 4.9 warns about that.

- fix signed vs. unsigned comparison warning:
    The in-memory table will luckily not even come close to the limits imposed
    by ssize_t. If it ever reaches that, we've got bigger problems.

src/regexp.c

@@ -6932,6 +6932,9 @@ static regengine_T bt_regengine =
 #ifdef REGEXP_DEBUG
   ,(char_u *)""
 #endif
+#ifdef DEBUG
+  ,NULL
+#endif
 };
 
 
@@ -6950,6 +6953,9 @@ static regengine_T nfa_regengine =
 #ifdef REGEXP_DEBUG
   ,(char_u *)""
 #endif
+#ifdef DEBUG
+  , NULL
+#endif
 };
 
 /* Which regexp engine to use? Needed for vim_regcomp().