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  <fwei...@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  <fwei...@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 ()