Fix gdb.threads/watchpoint-fork*.c compilation

This testcase currently fails to compile on Fedora 23:

 .../src/gdb/testsuite/gdb.threads/watchpoint-fork-mt.c: In function 'start':
 .../src/gdb/testsuite/gdb.threads/watchpoint-fork-mt.c:70:11: warning: implicit declaration of function 'pthread_yield' [-Wimplicit-function-declaration]
	i = pthread_yield ();
	    ^
 .../src/gdb/testsuite/gdb.threads/watchpoint-fork-child.c: In function 'forkoff':
 .../src/gdb/testsuite/gdb.threads/watchpoint-fork-child.c:114:8: warning: implicit declaration of function 'pthread_yield' [-Wimplicit-function-declaratio
 n]
     i = pthread_yield ();
	 ^
 /tmp/ccUkNIsI.o: In function `start':
 .../src/gdb/testsuite/gdb.threads/watchpoint-fork-mt.c:70: undefined reference to `pthread_yield'
 (...)
 collect2: error: ld returned 1 exit status

 UNSUPPORTED: gdb.threads/watchpoint-fork.exp: child: multithreaded: Couldn't compile watchpoint-fork-child.c: unrecognized error
 UNTESTED: gdb.threads/watchpoint-fork.exp: child: multithreaded: watchpoint-fork.exp
 testcase .../src/gdb/testsuite/gdb.threads/watchpoint-fork.exp completed i

The glibc manual says, on _GNU_SOURCE:

 "You should define these macros by using ‘#define’ preprocessor
 directives at the top of your source code files. These directives must
 come before any #include of a system header file."

I instead put it in the header all the .c files of the testcase must
include anyway.

gdb/testsuite/ChangeLog:
2016-03-01  Pedro Alves  <palves@redhat.com>

	* gdb.threads/watchpoint-fork-child.c: Include "watchpoint-fork.h"
	before anything else.
	* gdb.threads/watchpoint-fork-mt.c: Likewise.  Don't define
	_GNU_SOURCE here.
	* gdb.threads/watchpoint-fork-st.c: Include "watchpoint-fork.h"
	before anything else.
	* gdb.threads/watchpoint-fork.h: Define _GNU_SOURCE.

gdb/testsuite/ChangeLog

@@ -1,3 +1,13 @@
+2016-03-01  Pedro Alves  <palves@redhat.com>
+
+	* gdb.threads/watchpoint-fork-child.c: Include "watchpoint-fork.h"
+	before anything else.
+	* gdb.threads/watchpoint-fork-mt.c: Likewise.  Don't define
+	_GNU_SOURCE here.
+	* gdb.threads/watchpoint-fork-st.c: Include "watchpoint-fork.h"
+	before anything else.
+	* gdb.threads/watchpoint-fork.h: Define _GNU_SOURCE.
+
 2016-03-01  Pedro Alves  <palves@redhat.com>
 
 	* gdb.base/catch-fork-kill.c: Include <sys/wait.h>.

gdb/testsuite/gdb.threads/watchpoint-fork-child.c

@@ -17,6 +17,8 @@
    You should have received a copy of the GNU General Public License
    along with this program; if not, see <http://www.gnu.org/licenses/>.  */
 
+#include "watchpoint-fork.h"
+
 #include <string.h>
 #include <errno.h>
 #include <unistd.h>
@@ -24,8 +26,6 @@
 #include <signal.h>
 #include <stdio.h>
 
-#include "watchpoint-fork.h"
-
 /* `pid_t' may not be available.  */
 
 static volatile int usr1_got;

gdb/testsuite/gdb.threads/watchpoint-fork-mt.c

@@ -17,22 +17,19 @@
    You should have received a copy of the GNU General Public License
    along with this program; if not, see <http://www.gnu.org/licenses/>.  */
 
+#include "watchpoint-fork.h"
+
 #include <assert.h>
 #include <unistd.h>
 #include <sys/wait.h>
 #include <stdio.h>
 #include <stdlib.h>
-
-/* pthread_yield is a GNU extension.  */
-#define _GNU_SOURCE
 #include <pthread.h>
 
 #include <asm/unistd.h>
 #include <unistd.h>
 #define gettid() syscall (__NR_gettid)
 
-#include "watchpoint-fork.h"
-
 /* Non-atomic `var++' should not hurt as we synchronize the threads by the STEP
    variable.  Hit-comments need to be duplicated there to catch both at-stops
    and behind-stops, depending on the target.  */

gdb/testsuite/gdb.threads/watchpoint-fork-st.c

@@ -17,14 +17,14 @@
    You should have received a copy of the GNU General Public License
    along with this program; if not, see <http://www.gnu.org/licenses/>.  */
 
+#include "watchpoint-fork.h"
+
 #include <assert.h>
 #include <unistd.h>
 #include <sys/wait.h>
 #include <stdio.h>
 #include <stdlib.h>
 
-#include "watchpoint-fork.h"
-
 volatile int var;
 
 void

gdb/testsuite/gdb.threads/watchpoint-fork.h

@@ -17,6 +17,9 @@
    You should have received a copy of the GNU General Public License
    along with this program; if not, see <http://www.gnu.org/licenses/>.  */
 
+/* pthread_yield is a GNU extension.  */
+#define _GNU_SOURCE
+
 #ifdef THREAD
 #include <pthread.h>