[cmake] Reintroduce (ldconfig-compatible) SOVERSIONs on shared libraries

[mgorny]

Summary

Reintroduce versioning of shared libraries via SOVERSION, addressing the issues with the previous design, since Gentoo is relying on shared-split install of LLVM. The SOVERSIONs were originally introduced in r229720 for all libraries, and removed in r252093 in favor of custom SONAME. As far as I understand, the major concern with the old versioning was that the used versions were incompatible with ldconfig.

Having considered that, this commit introduce SOVERSIONS with the following considerations:

1. SOVERSIONs are formed of major & minor version concatenated -- i.e. for 4.0 its .so.40. This matches the common practice where the first version number indicates ABI breakage, and therefore fixes the issues

with ldconfig. Additionally, VERSION with the remaining verion components appended is used, however this is not strictly necessary.

2. The versioning is only applied to libraries with no explicit SONAME specified -- i.e. it won't apply to libLLVM but only to the split libraries. It will also apply to libraries installed by the subprojects.

3. The versioning is only done on *nix systems, Darwin excluded. This matches the current use of SONAME.

[mgorny]

Gentle ping. I would really like to get this settled before we unleash LLVM 3.9.0 on Gentoo since we need SONAMEs, and I would rather avoid continuing to use custom SONAMEs if we could agree on something.

[beanz]

LGTM.

[mgorny]

r283189, thanks.

cmake/modules/AddLLVM.cmake

function(llvm_add_library name)
   endif()
 
   if(ARG_SHARED)
     if(WIN32)
       set_target_properties(${name} PROPERTIES
         PREFIX ""
         )
     endif()
+
+  # Set SOVERSION on shared libraries that lack explicit SONAME
+  # specifier, on *nix systems that are not Darwin.
+    if(UNIX AND NOT APPLE AND NOT ARG_SONAME)
+      set_target_properties(${name}
+        PROPERTIES
+    # Concatenate the version numbers since ldconfig expects exactly
+    # one component indicating the ABI version, while LLVM uses
+    # major+minor for that.
+        SOVERSION ${LLVM_VERSION_MAJOR}${LLVM_VERSION_MINOR}
+        VERSION ${LLVM_VERSION_MAJOR}${LLVM_VERSION_MINOR}.${LLVM_VERSION_PATCH}${LLVM_VERSION_SUFFIX})
+    endif()
   endif()
 
   if(ARG_MODULE OR ARG_SHARED)
     # Do not add -Dname_EXPORTS to the command-line when building files in this
     # target. Doing so is actively harmful for the modules build because it
     # creates extra module variants, and not useful because we don't use these
     # macros.
     set_target_properties( ${name} PROPERTIES DEFINE_SYMBOL "" )