res_resolver_unbound: Allow compilation with libunbound version < 1.5

libunbound at version 1.4.20 (which CentOS still uses) declared all
of their string function parameters as as 'char *'.  1.4.21 changed
them all to 'const char *'.  Thankfully 1.4.21 also introduced the
UNBOUND_VERSION_MAJOR define so configure now checks for that and
sets HAVE_UNBOUND_CONST_PARAMS.  res_resolver_unbound then checks
that and casts away the 'const' if it's not set.

Tested compile and testsuite on CentOS6 (1.4.20), Ubuntu14 (1.4.22) and
Fedora24 (1.5.4).  There are a few failing tests to be addressed though.

ASTERISK-26283 #close

Change-Id: Ib708b19b706c5d0ba7b7d5473e6df339d9ae4148

configure

@@ -23683,7 +23683,7 @@ if test "x${PBX_UNBOUND}" != "x1" -a "${USE_UNBOUND}" != "no"; then
          pbxlibdir="-L${UNBOUND_DIR}"
       fi
    fi
-   pbxfuncname="ub_ctx_add_ta_autr"
+   pbxfuncname="ub_ctx_delete"
    if test "x${pbxfuncname}" = "x" ; then   # empty lib, assume only headers
       AST_UNBOUND_FOUND=yes
    else
@@ -23777,6 +23777,49 @@ fi
 
 
 
+    if test "x${PBX_UNBOUND_CONST_PARAMS}" != "x1" -a "${USE_UNBOUND_CONST_PARAMS}" != "no"; then
+        { $as_echo "$as_me:${as_lineno-$LINENO}: checking for UNBOUND_VERSION_MAJOR declared in unbound.h" >&5
+$as_echo_n "checking for UNBOUND_VERSION_MAJOR declared in unbound.h... " >&6; }
+        saved_cppflags="${CPPFLAGS}"
+        if test "x${UNBOUND_CONST_PARAMS_DIR}" != "x"; then
+            UNBOUND_CONST_PARAMS_INCLUDE="-I${UNBOUND_CONST_PARAMS_DIR}/include"
+        fi
+        CPPFLAGS="${CPPFLAGS} ${UNBOUND_CONST_PARAMS_INCLUDE}"
+
+        cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+ #include <unbound.h>
+int
+main ()
+{
+#if !defined(UNBOUND_VERSION_MAJOR)
+                                    (void) UNBOUND_VERSION_MAJOR;
+                                #endif
+
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_compile "$LINENO"; then :
+     { $as_echo "$as_me:${as_lineno-$LINENO}: result: yes" >&5
+$as_echo "yes" >&6; }
+                PBX_UNBOUND_CONST_PARAMS=1
+
+$as_echo "#define HAVE_UNBOUND_CONST_PARAMS 1" >>confdefs.h
+
+
+
+else
+     { $as_echo "$as_me:${as_lineno-$LINENO}: result: no" >&5
+$as_echo "no" >&6; }
+
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+
+        CPPFLAGS="${saved_cppflags}"
+    fi
+
+
 
 if test "x${PBX_UNIXODBC}" != "x1" -a "${USE_UNIXODBC}" != "no"; then
    pbxlibdir=""