asterisk11: clean up compile stage

asterisk11: clean up compile stage

- Don't add extra TARGET_LDFLAGS for asterisk13-pbx-lua. It is not
  needed anymore.
- Stop setting -Wl,-rpath-link in LDFLAGS. There seems to be no point to
  do that (anymore).
- Stop calling specific make targets like version.h; make handles the
  build properly without it.
- Add AST_FORTIFY_SOURCE to MAKE_FLAGS (seen in Debian rules file,
  prevents asterisk's build system to mess with OpenWrt/LEDE flags).

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>