[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

llvm-svn: 250469
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 1ab69fc5321336938e7cb0d960a13ab26395d8d5
diff --git a/docs/BuildingLibcxx.rst b/docs/BuildingLibcxx.rst
index 6e45578..a7366c5 100644
--- a/docs/BuildingLibcxx.rst
+++ b/docs/BuildingLibcxx.rst
@@ -170,6 +170,14 @@
   If this option is enabled, libc++ will try and link the selected ABI library
   statically.
 
+.. option:: LIBCXX_ENABLE_ABI_LINKER_SCRIPT:BOOL
+
+  **Default**: ``ON`` by default on UNIX platforms other than Apple unless
+  'LIBCXX_ENABLE_STATIC_ABI_LIBRARY' is ON. Otherwise the default value is ``OFF``.
+
+  This option generate and installs a linker script as ``libc++.so`` which
+  links the correct ABI library.
+
 .. option:: LIBCXXABI_USE_LLVM_UNWINDER:BOOL
 
   **Default**: ``OFF``