Turn on -Werror=implicit-function-declaration

GCC 4.4, the minimum compiler version, supports this option.  Unlike
other warnings, -Wimplicit-function-declaration warnings should be
independent of compiler versions, so this change should not cause
compiler-specific build failures.

ChangeLog

@@ -1,3 +1,11 @@
+2014-09-10  Florian Weimer  <fweimer@redhat.com>
+
+	* Makeconfig (gccwarn-c): Turn implicit-function-declaration
+	warnings into errors.
+
+	* stdlib/tst-tls-atexit-lib.c: Include <stdlib.h>, for the
+	__cxa_thread_atexit_impl prototype.
+
 2014-09-09  Steve Ellcey  <sellcey@mips.com>
 
 	* sysdeps/mips/preconfigure: Put code inside mips* case statement.

Makeconfig

@@ -717,7 +717,7 @@ else
 +gccwarn := -Wall -Wwrite-strings -Winline
 endif
 +gccwarn += -Wundef
-+gccwarn-c = -Wstrict-prototypes
++gccwarn-c = -Wstrict-prototypes -Werror=implicit-function-declaration
 
 # We do not depend on the address of constants in different files to be
 # actually different, so allow the compiler to merge them all.

stdlib/tst-tls-atexit-lib.c

@@ -16,6 +16,8 @@
    License along with the GNU C Library; if not, see
    <http://www.gnu.org/licenses/>.  */
 
+#include <stdlib.h>
+
 extern void *__dso_handle;
 
 typedef struct