[CMake] Bug 25059 - CMake libllvm.so.$MAJOR.$MINOR shared object name not compatible with ldconfig

Summary:
This change makes the CMake build system generate libraries for Linux and Darwin matching the makefile build system.

Linux libraries follow the pattern lib${name}.${MAJOR}.${MINOR}.so so that ldconfig won't pick it up incorrectly.

Darwin libraries are not versioned.

Note: On linux the non-versioned symlink is generated at install-time not build time. I plan to fix that eventually, but I expect that is good enough for the purposes of fixing this bug.

Reviewers: loladiro, tstellarAMD

Subscribers: axw, llvm-commits

Differential Revision: http://reviews.llvm.org/D13841

llvm-svn: 252093