Use setenv(), fix warnings

- Fix -Wundef -Wold-style-definition warnings
  - Make pll_free() static

[jc: original patch by Timo had another unrelated bits:

  - Use setenv() instead of putenv()

 I'm postponing that part for now.]

Signed-off-by: Timo Hirvonen <tihirvon@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>

exec_cmd.c

@@ -13,7 +13,7 @@ void git_set_exec_path(const char *exec_path)
 
 
 /* Returns the highest-priority, location to look for git programs. */
-const char *git_exec_path()
+const char *git_exec_path(void)
 {
 	const char *env;