Prevent CMake from failing when no libcpp paths are found.

This patch instead prints an error with some useful debugging
information that should help identify the cause of this issue.

CMakeLists.txt

@@ -213,12 +213,16 @@ if (NOT libcxx)
   # Only use the first path from the HEADERS_LOCATION (which is separated by colons).
   get_property(__libcpp_full_paths GLOBAL PROPERTY ROOT_CLING_CXX_HEADERS_LOCATION)
   string(REGEX MATCHALL "[^:]+" __libcpp_full_paths_list "${__libcpp_full_paths}")
-  list(GET __libcpp_full_paths_list 0 __libcpp_full_path)
+  if (__libcpp_full_paths_list)
+    list(GET __libcpp_full_paths_list 0 __libcpp_full_path)
 
-  configure_file(${CMAKE_SOURCE_DIR}/build/unix/modulemap.overlay.yaml.in ${CMAKE_BINARY_DIR}/include/modulemap.overlay.yaml @ONLY)
+    configure_file(${CMAKE_SOURCE_DIR}/build/unix/modulemap.overlay.yaml.in ${CMAKE_BINARY_DIR}/include/modulemap.overlay.yaml @ONLY)
 
-  configure_file(${CMAKE_SOURCE_DIR}/build/unix/stl.cppmap ${CMAKE_BINARY_DIR}/include/stl.cppmap)
-  configure_file(${CMAKE_SOURCE_DIR}/build/unix/libc.modulemap ${CMAKE_BINARY_DIR}/include/libc.modulemap)
+    configure_file(${CMAKE_SOURCE_DIR}/build/unix/stl.cppmap ${CMAKE_BINARY_DIR}/include/stl.cppmap)
+    configure_file(${CMAKE_SOURCE_DIR}/build/unix/libc.modulemap ${CMAKE_BINARY_DIR}/include/libc.modulemap)
+  else()
+    message(SEND_ERROR "Couldn't find c++ library paths, no modulemap overlay will be installed! (__libcpp_full_paths = '${__libcpp_full_paths}')")
+  endif()
 endif()
 
 if (cxxmodules)