Const correctness fixes for darwin-driver.c

[Jeff Law]

The various darwin ports in config-list.mk fail to build with a trunk 
compiler due to two minor const-correctness issues.

With this patch installed all the darwin ports in config-list.mk build 
successfully.

Installed on the trunk.

Jeff

diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index 31aac2b..698277a 100644
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,3 +1,8 @@
+2017-01-02  Jeff Law  <l...@redhat.com>
+
+       * config/darwin-driver.c (darwin_driver_init): Const-correctness
+       fixes for first_period and second_period variables.
+
 2017-01-02  Uros Bizjak  <ubiz...@gmail.com>
 
        PR target/78967
diff --git a/gcc/config/darwin-driver.c b/gcc/config/darwin-driver.c
index 0c4f0cd..e3ed79d 100644
--- a/gcc/config/darwin-driver.c
+++ b/gcc/config/darwin-driver.c
@@ -299,10 +299,10 @@ darwin_driver_init (unsigned int *decoded_options_count,
   if (vers_string != NULL)
     {
       char *asm_major = NULL;
-      char *first_period = strchr(vers_string, '.');
+      const char *first_period = strchr(vers_string, '.');
       if (first_period != NULL)
        {
-         char *second_period = strchr(first_period+1, '.');
+         const char *second_period = strchr(first_period+1, '.');
          if (second_period  != NULL)
            asm_major = xstrndup (vers_string, second_period-vers_string);
          else