treewide: revise library packaging

- Annotate versionless libraries (such as libubox, libuci etc.) with a fixed
  ABI_VERSION resembling the source date of the last incompatible change
- Annotate packages shipping versioned library objects with ABI_VERSION
- Stop shipping unversioned library symlinks for packages with ABI_VERSION

Ref: https://openwrt.org/docs/guide-developer/package-policies#shared_libraries
Ref: https://github.com/KanjiMonster/maintainer-tools/blob/master/check-abi-versions.pl
Signed-off-by: Jo-Philipp Wich <jo@mein.io>

package/libs/libtool/Makefile

@@ -9,7 +9,7 @@ include $(TOPDIR)/rules.mk
 
 PKG_NAME:=libtool
 PKG_VERSION:=2.4.6
-PKG_RELEASE:=1
+PKG_RELEASE:=2
 
 PKG_SOURCE:=$(PKG_NAME)-$(PKG_VERSION).tar.xz
 PKG_SOURCE_URL:=@GNU/libtool
@@ -31,6 +31,7 @@ define Package/libltdl
   CATEGORY:=Libraries
   TITLE:=A generic dynamic object loading library
   URL:=http://www.gnu.org/software/libtool/
+  ABI_VERSION:=7
 endef
 
 define Build/InstallDev