[CMake] Create a separate install target for libcxx headers

This change doesn't impact the behavior of the install-libcxx target which installs whichever libcxx components you build, it just adds a separate target to just install the headers.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@268124 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CMakeLists.txt

@@ -180,8 +180,11 @@ if (NOT CMAKE_CONFIGURATION_TYPES AND (LIBCXX_INSTALL_LIBRARY OR
     if(LIBCXX_INSTALL_LIBRARY)
       set(deps DEPENDS cxx)
     endif()
+    if(LIBCXX_INSTALL_HEADERS)
+      set(deps DEPENDS install-libcxx-headers)
+    endif()
     add_custom_target(install-libcxx
-                      ${deps}
+                      ${deps} 
                       COMMAND "${CMAKE_COMMAND}"
                       -DCMAKE_INSTALL_COMPONENT=libcxx
                       -P "${LIBCXX_BINARY_DIR}/cmake_install.cmake")