kea: fix compilation with libcxx

Boost headers try to include experimental/string_view when std is less
than c++17. This does not work ith libcxx where this header is not
present.

Refreshed patches.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

net/kea/patches/001-fix-cross-compile.patch

@@ -1,6 +1,8 @@
 --- a/configure.ac
 +++ b/configure.ac
-@@ -542,8 +542,8 @@ AC_TRY_COMPILE([
+@@ -580,10 +580,10 @@ AC_TRY_COMPILE([
+ 
+ usable_regex=
  AC_MSG_CHECKING(for usuable C++11 regex)
 -AC_TRY_RUN([
 +AC_TRY_COMPILE([