Add workaround for missing 'restrict' keyword.

Add a cpp #define that removes 'restrict' keyword usage unless the
compiler definitely supports C99.  As written, 'restrict' is only
enabled if the compiler supports the -std=gnu99 option (e.g. gcc and
llvm).

Reported by Tobias Hieta.

configure.ac

@@ -19,7 +19,9 @@ AC_COMPILE_IFELSE([AC_LANG_PROGRAM(
 ]], [[
     return 0;
 ]])],
+              [je_cv_cflags_appended=$1]
               AC_MSG_RESULT([yes]),
+              [je_cv_cflags_appended=]
               AC_MSG_RESULT([no])
               [CFLAGS="${TCFLAGS}"]
 )
@@ -128,6 +130,9 @@ if test "x$CFLAGS" = "x" ; then
   no_CFLAGS="yes"
   if test "x$GCC" = "xyes" ; then
     JE_CFLAGS_APPEND([-std=gnu99])
+    if test "x$je_cv_cflags_appended" = "x-std=gnu99" ; then
+      AC_DEFINE_UNQUOTED([JEMALLOC_HAS_RESTRICT])
+    fi
     JE_CFLAGS_APPEND([-Wall])
     JE_CFLAGS_APPEND([-pipe])
     JE_CFLAGS_APPEND([-g3])