Add a shared library for LLVM, named libLLVM2.7svn.(so|dylib), and add an

Add a shared library for LLVM, named libLLVM2.7svn.(so|dylib), and add an
--enable-shared configure flag to have the tools linked shared. (2.7svn is just
$(LLVMVersion) so it'll change to "2.7" in the release.)  Always link the
example programs shared to test that the shared library keeps working.

On my mac laptop, Debug libLLVM2.7svn.dylib is 39MB, and opt (for example) is
16M static vs 440K shared.

Two things are less than ideal here:
1) The library doesn't include any version information. Since we expect to break
the ABI with every release, this shouldn't be much of a problem. If we do
release a compatible 2.7.1, we may be able to hack its library to work with
binaries compiled against 2.7.0, or we can just ask them to recompile. I'm
hoping to get a real packaging expert to look at this for the 2.8 release.
2) llvm-config doesn't yet have an option to print link options for the shared
library. I'll add this as a subsequent patch.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@96559 91177308-0d34-0410-b5e6-96231b3b80d8