[libcxx] Make libc++.so a linker script by default on most platforms.

Summary:
This patch turns on `LIBCXX_ENABLE_ABI_LINKER_SCRIPT` by default whenever `LLVM_HAVE_LINK_VERSION_SCRIPT` is ON. This turns out to be whenever:

1. WIN32 is not defined.
2 UNIX is defined.
3. APPLE is not defined.

While `LLVM_HAVE_LINK_VERSION_SCRIPT` is meant to reflect exactly what we are asking I think it's close enough.

After committing this patch Linux users will no longer have to use "-lc++abi" explicitly!




Reviewers: mclow.lists, danalbert, compnerd, jroelofs

Subscribers: emaste, rengolin, cbergstrom, cfe-commits

Differential Revision: http://reviews.llvm.org/D13739

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@250469 91177308-0d34-0410-b5e6-96231b3b80d8

docs/UsingLibcxx.rst

@@ -54,10 +54,10 @@ An example of using ``LD_LIBRARY_PATH``:
 Using libc++ on Linux
 =====================
 
-On Linux libc++ typically links to a shared version of libc++abi. Unfortunately
-you can't simply run clang with "-stdlib=libc++" as clang is not set up to
-link for this configuration. To get around this you'll have to manually
-link libc++abi yourself. For example:
+On Linux libc++ can typically be used with only '-stdlib=libc++'. However
+some libc++ installations require the user manually link libc++abi themselves.
+If you are running into linker errors when using libc++ try adding '-lc++abi'
+to the link line.  For example:
 
 .. code-block:: bash