[libc++] Improve CMake include directory search.

This patch has been tested in D70631, but it should be reviewed
separately.

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D114248

GitOrigin-RevId: cc9fdedba7c9b19fed24a7b67f63a3a2e7a729bd
diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index d52c530..c0e87bb 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -197,6 +197,7 @@
 # Build the shared library.
 if (LIBCXX_ENABLE_SHARED)
   add_library(cxx_shared SHARED ${exclude_from_all} ${LIBCXX_SOURCES} ${LIBCXX_HEADERS})
+  target_include_directories(cxx_shared PRIVATE ${CMAKE_CURRENT_SOURCE_DIR})
   target_link_libraries(cxx_shared PUBLIC cxx-headers
                                    PRIVATE ${LIBCXX_LIBRARIES})
   set_target_properties(cxx_shared
@@ -275,6 +276,7 @@
 # Build the static library.
 if (LIBCXX_ENABLE_STATIC)
   add_library(cxx_static STATIC ${exclude_from_all} ${LIBCXX_SOURCES} ${LIBCXX_HEADERS})
+  target_include_directories(cxx_static PRIVATE ${CMAKE_CURRENT_SOURCE_DIR})
   target_link_libraries(cxx_static PUBLIC cxx-headers
                                    PRIVATE ${LIBCXX_LIBRARIES})
   set_target_properties(cxx_static