Index: CMakeLists.txt
===================================================================
--- CMakeLists.txt
+++ CMakeLists.txt
@@ -55,8 +55,17 @@
 option(LIBCXX_INCLUDE_DOCS "Build the libc++ documentation." ${LLVM_INCLUDE_DOCS})
 set(LIBCXX_LIBDIR_SUFFIX "${LLVM_LIBDIR_SUFFIX}" CACHE STRING
     "Define suffix of library directory name (32/64)")
-option(LIBCXX_INSTALL_HEADERS "Install the libc++ headers." ON)
-option(LIBCXX_INSTALL_LIBRARY "Install the libc++ library." ON)
+
+set(INSTALL_HEADERS_DEFAULT ON)
+set(INSTALL_LIBRARY_DEFAULT ON)
+if(${CMAKE_SYSTEM_NAME} MATCHES "Darwin"
+  AND "${CMAKE_INSTALL_PREFIX}" STREQUAL "/usr")
+  set(INSTALL_HEADERS_DEFAULT OFF)
+  set(INSTALL_LIBRARY_DEFAULT OFF)
+endif()
+option(LIBCXX_INSTALL_HEADERS "Install the libc++ headers." ${INSTALL_HEADERS_DEFAULT})
+option(LIBCXX_INSTALL_LIBRARY "Install the libc++ library." ${INSTALL_LIBRARY_DEFAULT})
+
 option(LIBCXX_INSTALL_SUPPORT_HEADERS "Install libc++ support headers." ON)
 set(LIBCXX_ABI_VERSION 1 CACHE STRING "ABI version of libc++.")
 option(LIBCXX_ABI_UNSTABLE "Unstable ABI of libc++." OFF)
@@ -133,23 +142,6 @@
 set(LIBCXX_COVERAGE_LIBRARY "" CACHE STRING
     "The Profile-rt library used to build with code coverage")
 
-# Don't allow a user to accidentally overwrite the system libc++ installation on Darwin.
-# If the user specifies -DCMAKE_INSTALL_PREFIX=/usr the install rules for libc++
-# will not be generated and a warning will be issued.
-option(LIBCXX_OVERRIDE_DARWIN_INSTALL "Enable overwriting darwins libc++ installation." OFF)
-mark_as_advanced(LIBCXX_OVERRIDE_DARWIN_INSTALL) # Don't show this option by default.
-
-if (${CMAKE_SYSTEM_NAME} MATCHES "Darwin" AND NOT LIBCXX_OVERRIDE_DARWIN_INSTALL)
-  if ("${CMAKE_INSTALL_PREFIX}" STREQUAL "/usr")
-    message(WARNING "Disabling libc++ install rules because installation would "
-                    "overwrite the systems installation. Configure with "
-                    "-DLIBCXX_OVERRIDE_DARWIN_INSTALL=ON to suppress this behaviour.")
-    mark_as_advanced(CLEAR LIBCXX_OVERRIDE_DARWIN_INSTALL) # Show the override option.
-    set(LIBCXX_INSTALL_HEADERS OFF)
-    set(LIBCXX_INSTALL_LIBRARY OFF)
-  endif()
-endif()
-
 #===============================================================================
 # Check option configurations
 #===============================================================================