Include version in name of shared libs

libLLVM.so -> libLLVM-$MAJOR.$MINOR.so

Source: http://pkgs.fedoraproject.org/cgit/rpms/llvm3.9.git/tree/llvm-soversion.patch?id=3dac83eaa5b88f550ae50125b14b8f644e10617b
--- a/cmake/modules/AddLLVM.cmake
+++ b/cmake/modules/AddLLVM.cmake
@@ -450,6 +450,18 @@
         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)