librc: mark stringlist functions as mallocs

This gives a hint to the compiler that allocations (return values)
from this function should be paired with a corresponding dealloc/free
function

In this case, it means that every rc_stringlist that rc_stringlist_new()
returns should eventually be freed by calling rc_stringlist_free(ptr)
where ptr is the relevant rc_stringlist.

We have to add a test for this into the build system
because only GCC supports this for now. In future, we might
be able to use meson's has_function_attribute (it does support
'malloc', just not AFAICT 'malloc with arguments').

Signed-off-by: Sam James <sam@gentoo.org>

src/librc/rc.h.in

@@ -528,8 +528,15 @@ bool rc_yesno(const char *);
 /*! @name String List functions
  * Every string list should be released with a call to rc_stringlist_free. */
 
-/*! Create a new stringlist
+/*! Frees each item on the list and the list itself.
+ * @param list to free */
+void rc_stringlist_free(RC_STRINGLIST *);
+
+/*! Create a new stringlinst
  * @return pointer to new list */
+#ifdef HAVE_MALLOC_EXTENDED_ATTRIBUTE
+__attribute__ ((malloc (rc_stringlist_free, 1)))
+#endif
 RC_STRINGLIST *rc_stringlist_new(void);
 
 /*! Duplicate the item, add it to end of the list and return a pointer to it.
@@ -568,10 +575,6 @@ RC_STRINGLIST *rc_stringlist_split(const char *, const char *);
  * @param list to sort */
 void rc_stringlist_sort(RC_STRINGLIST **);
 
-/*! Frees each item on the list and the list itself.
- * @param list to free */
-void rc_stringlist_free(RC_STRINGLIST *);
-
 typedef struct rc_pid
 {
 	pid_t pid;