Fix GCC_LINUX_FUTEX to work with C99 compilers

[Florian Weimer]

Without this change, libstdc++ is built without futex symbols if GCC
rejects implicit function declarations in default mode.

Thanks,
Florian

config/ChangeLog:

2019-09-06  Florian Weimer  <fweimer@redhat.com>

	* futex.m4 (GCC_LINUX_FUTEX): Include <unistd.h> for the syscall
	function.

libgomp/ChangeLog, libitm/ChangeLog, libstdc++-v3/ChangeLog:

2019-09-06  Florian Weimer  <fweimer@redhat.com>

	* configure: Regenerate.

Index: config/futex.m4
===================================================================
--- config/futex.m4	(revision 275450)
+++ config/futex.m4	(working copy)
@@ -22,6 +22,7 @@
 	AC_LINK_IFELSE(
 	 [AC_LANG_PROGRAM(
 	  [#include <sys/syscall.h>
+	   #include <unistd.h>
 	   int lk;],
 	  [syscall (SYS_gettid); syscall (SYS_futex, &lk, 0, 0, 0);])],
 	  [save_LIBS="$LIBS"
@@ -48,6 +49,7 @@
 	AC_LINK_IFELSE(
 	 [AC_LANG_PROGRAM(
 	  [#include <sys/syscall.h>
+	   #include <unistd.h>
 	   int lk;],
 	  [syscall (SYS_gettid); syscall (SYS_futex, &lk, 0, 0, 0);])],[],
 	  [AC_MSG_ERROR([SYS_gettid and SYS_futex required for --enable-linux-futex])])
Index: libgomp/configure
===================================================================
--- libgomp/configure	(revision 275450)
+++ libgomp/configure	(working copy)
@@ -15888,6 +15888,7 @@
 	cat confdefs.h - <<_ACEOF >conftest.$ac_ext
 /* end confdefs.h.  */
 #include <sys/syscall.h>
+	   #include <unistd.h>
 	   int lk;
 int
 main ()
@@ -15940,6 +15941,7 @@
 	cat confdefs.h - <<_ACEOF >conftest.$ac_ext
 /* end confdefs.h.  */
 #include <sys/syscall.h>
+	   #include <unistd.h>
 	   int lk;
 int
 main ()
Index: libitm/configure
===================================================================
--- libitm/configure	(revision 275450)
+++ libitm/configure	(working copy)
@@ -16481,6 +16481,7 @@
 	cat confdefs.h - <<_ACEOF >conftest.$ac_ext
 /* end confdefs.h.  */
 #include <sys/syscall.h>
+	   #include <unistd.h>
 	   int lk;
 int
 main ()
@@ -16533,6 +16534,7 @@
 	cat confdefs.h - <<_ACEOF >conftest.$ac_ext
 /* end confdefs.h.  */
 #include <sys/syscall.h>
+	   #include <unistd.h>
 	   int lk;
 int
 main ()
Index: libstdc++-v3/configure
===================================================================
--- libstdc++-v3/configure	(revision 275450)
+++ libstdc++-v3/configure	(working copy)
@@ -72670,6 +72670,7 @@
 cat confdefs.h - <<_ACEOF >conftest.$ac_ext
 /* end confdefs.h.  */
 #include <sys/syscall.h>
+	   #include <unistd.h>
 	   int lk;
 int
 main ()
@@ -72728,6 +72729,7 @@
 cat confdefs.h - <<_ACEOF >conftest.$ac_ext
 /* end confdefs.h.  */
 #include <sys/syscall.h>
+	   #include <unistd.h>
 	   int lk;
 int
 main ()