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/term.c

@@ -2225,12 +2225,14 @@ static void term_color(char_u *s, int n)
       && (STRCMP(s + i + 1, "%p1%dm") == 0
           || STRCMP(s + i + 1, "%dm") == 0)
       && (s[i] == '3' || s[i] == '4')) {
-    sprintf(buf,
+    const char *fmt =
 #ifdef TERMINFO
-        "%s%s%%p1%%dm",
+        "%s%s%%p1%%dm";
 #else
-        "%s%s%%dm",
+        "%s%s%%dm";
 #endif
+    sprintf(buf,
+        fmt,
         i == 2 ? IF_EB("\033[", ESC_STR "[") : "\233",
         s[i] == '3' ? (n >= 16 ? "38;5;" : "9")
         : (n >= 16 ? "48;5;" : "10"));