[commit/obvious] Use '::iterator' instead of '::const_iterator' on environ.c (and fix breakage on early versions of libstdc++)

[Sergio Durigan Junior]

Even though C++11 supports modifying containers using a const_iterator
(e.g., calling the 'erase' method of a std::vector), early versions of
libstdc++ did not implement that.  Some of our buildslaves are using
these versions (e.g., the AArch64 buildslave uses gcc 4.8.8), and my
previous commit causes a breakage on them.  The solution is simple:
just use a normal iterator, without const.

gdb/ChangeLog:
2017-06-20  Sergio Durigan Junior  <sergiodj@redhat.com>

	* common/environ.c (gdb_environ::unset): Use '::iterator' instead
	of '::const_iterator'.
---
 gdb/ChangeLog        | 5 +++++
 gdb/common/environ.c | 4 ++--
 2 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 77001fa..f4e2b08 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,5 +1,10 @@
 2017-06-20  Sergio Durigan Junior  <sergiodj@redhat.com>
 
+	* common/environ.c (gdb_environ::unset): Use '::iterator' instead
+	of '::const_iterator'.
+
+2017-06-20  Sergio Durigan Junior  <sergiodj@redhat.com>
+
 	* Makefile.in (SUBDIR_UNITTESTS_SRCS): Add
 	'unittests/environ-selftests.c'.
 	(SUBDIR_UNITTESTS_OBS): Add 'environ-selftests.o'.
diff --git a/gdb/common/environ.c b/gdb/common/environ.c
index 20f8aba..2d13957 100644
--- a/gdb/common/environ.c
+++ b/gdb/common/environ.c
@@ -116,8 +116,8 @@ gdb_environ::unset (const char *var)
 
   /* We iterate until '.cend () - 1' because the last element is
      always NULL.  */
-  for (std::vector<char *>::const_iterator el = m_environ_vector.cbegin ();
-       el != m_environ_vector.cend () - 1;
+  for (std::vector<char *>::iterator el = m_environ_vector.begin ();
+       el != m_environ_vector.end () - 1;
        ++el)
     if (match_var_in_string (*el, var, len))
       {
-- 
2.9.3