asterisk13: 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 seems to be no point to
do that (anymore).
- 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).
Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>