[libc++] Prevent stale site configuration headers

[smeenai]

Summary

If we define cmake macros that require a site config, and then undefine
all such macros, a stale site config header will be left behind.
Explicitly delete any generate site config if we don't need one to avoid
this.

[smeenai]

Ping.

[mclow.lists]

This looks reasonable to me.

[EricWF]

CMakeLists.txt

633 ↗

(On Diff #111096)

Maybe print a warning or a message here? While it seems useful to re-configure and remove the stale header without requiring the user delete the build directory, it does seem like an odd thing to do, at least silently.

Since this shouldn't be happening very often, I don't think it would be too noisy to explicitly warn the user what is going on.

[smeenai]

CMakeLists.txt

633 ↗

(On Diff #111096)

Sounds good.

libcxx/trunk/CMakeLists.txt

 # By default libc++ on Windows expects to use a shared library, which requires
 # the headers to use DLL import/export semantics. However when building a
 # static library only we modify the headers to disable DLL import/export.
 if (DEFINED WIN32 AND LIBCXX_ENABLE_STATIC AND NOT LIBCXX_ENABLE_SHARED)
   message(STATUS "Generating custom __config for non-DLL Windows build")
   config_define(ON _LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS)
 endif()
 
+set(site_config_path "${LIBCXX_BINARY_DIR}/__config_site")
 if (LIBCXX_NEEDS_SITE_CONFIG)
   configure_file("include/__config_site.in"
-                 "${LIBCXX_BINARY_DIR}/__config_site"
+                 "${site_config_path}"
                  @ONLY)
 
   # Provide the config definitions by included the generated __config_site
   # file at compile time.
   if("${CMAKE_CXX_COMPILER_ID}" STREQUAL "MSVC" OR "${CMAKE_CXX_SIMULATE_ID}" STREQUAL "MSVC")
     add_compile_flags("/FI\"${LIBCXX_BINARY_DIR}/__config_site\"")
   else()
     add_compile_flags("-include ${LIBCXX_BINARY_DIR}/__config_site")
   endif()
+else()
+  if (EXISTS "${site_config_path}")
+    message(STATUS "Removing stale site configuration ${site_config_path}")
+    file(REMOVE "${site_config_path}")
+  endif()
 endif()
 
 #===============================================================================
 # Setup Source Code And Tests
 #===============================================================================
 include_directories(include)
 add_subdirectory(include)
 add_subdirectory(lib)