Force "restrict" not to be used when compiling with xlc.

Per buildfarm animal Hornet and followup manual testing by Noah Misch,
it appears xlc miscompiles code using "restrict" in at least some
cases. Allow disabling restrict usage with FORCE_DISABLE_RESTRICT=yes
in template files, and do so for aix/xlc.

Author: Andres Freund and Tom Lane
Discussion: https://postgr.es/m/1820.1507918762@sss.pgh.pa.us

configure

@@ -12466,6 +12466,10 @@ fi
 # spelling it understands, because it conflicts with
 # __declspec(restrict). Therefore we define pg_restrict to the
 # appropriate definition, which presumably won't conflict.
+#
+# Allow platforms with buggy compilers to force restrict to not be
+# used by setting $FORCE_DISABLE_RESTRICT=yes in the relevant
+# template.
 { $as_echo "$as_me:${as_lineno-$LINENO}: checking for C/C++ restrict keyword" >&5
 $as_echo_n "checking for C/C++ restrict keyword... " >&6; }
 if ${ac_cv_c_restrict+:} false; then :
@@ -12512,7 +12516,7 @@ _ACEOF
  ;;
  esac
 
-if test "$ac_cv_c_restrict" = "no" ; then
+if test "$ac_cv_c_restrict" = "no" -o "x$FORCE_DISABLE_RESTRICT" = "xyes"; then
   pg_restrict=""
 else
   pg_restrict="$ac_cv_c_restrict"

configure.in

@@ -1329,8 +1329,12 @@ PGAC_TYPE_LOCALE_T
 # spelling it understands, because it conflicts with
 # __declspec(restrict). Therefore we define pg_restrict to the
 # appropriate definition, which presumably won't conflict.
+#
+# Allow platforms with buggy compilers to force restrict to not be
+# used by setting $FORCE_DISABLE_RESTRICT=yes in the relevant
+# template.
 AC_C_RESTRICT
-if test "$ac_cv_c_restrict" = "no" ; then
+if test "$ac_cv_c_restrict" = "no" -o "x$FORCE_DISABLE_RESTRICT" = "xyes"; then
   pg_restrict=""
 else
   pg_restrict="$ac_cv_c_restrict"

src/template/aix

@@ -10,6 +10,10 @@ if test "$GCC" != yes ; then
       CFLAGS="-O2 -qmaxmem=16384 -qsrcmsg"
       ;;
   esac
+
+  # Due to a compiler bug, see 20171013023536.GA492146@rfd.leadboat.com for details,
+  # force restrict not to be used when compiling with xlc.
+  FORCE_DISABLE_RESTRICT=yes
 fi
 
 # Native memset() is faster, tested on: