asterisk-13.x: clean up compile stage

asterisk-13.x: clean up compile stage

- Don't add extra TARGET_LDFLAGS for asterisk13-pbx-lua. It is not
  needed anymore (apart from that the module isn't available currently,
  see next commit).
- Stop setting -Wl,-rpath-link in LDFLAGS. There is no point to do that.
- Stop calling specific make targets like version.h; make handles the
  build properly without it.
- Use the default compile routine instead of the custom one.
- Add AST_FORTIFY_SOURCE to MAKE_FLAGS (seen in Debian rules file,
  prevents asterisk's build system to mess with OpenWrt/LEDE flags).
- Remove $(SITE_VARS) (unused)
- Remove $(SDK) from ifneq. The variable prevents the ifneq from
  working on the buildbots. $(SDK) is set there, so the ifneqs that test
  for empty will always be true.

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