Added #if for the Linker.h header.
Remove previous fix, move it to Linker.hsc.

Also have to introduce a few constants to make up for the fact that
the llvm-c/Linker.h file is a relatively new addition.
Merge remote-tracking branch 'upstream/master'
Accommodating pure shared library LLVM installations.

The existing code always needs libLLVMCore to be around. This is sometimes
not the case, i.e., when only the monolithic shared library is installed:
libLLVM-$version.so.

I think this code will allow that case now, whereas I couldn't build it before
with Fedora's llvm package.
Merge pull request #72 from scottgw/master

Build fixes