cmake: Use `configure_file()` to implement forced C++ linkage

This finally prevents Ninja from rebuilding everything needlessly each time a
CMake reconfiguration was triggered. The root cause is that we used
`file(WRITE ...)`, which always unconditionally overwrites, so Ninja sees
those files as "dirty".

PiperOrigin-RevId: 453849514
Change-Id: Ib878df21371387baa7bf791a0a054e1ea5d6b6ae

CMakeLists.txt

@@ -90,9 +90,10 @@ configure_file(cmake/libcap_capability.h.in
 
 # Library with basic project settings. The empty file is there to be able to
 # define header-only libraries without cumbersome target_sources() hacks.
-file(WRITE "${SAPI_BINARY_DIR}/sapi_base_force_cxx_linkage.cc" "")
+configure_file(cmake/sapi_force_cxx_linkage.cc.in
+               "${SAPI_BINARY_DIR}/sapi_force_cxx_linkage.cc" COPYONLY)
 add_library(sapi_base STATIC
-  "${SAPI_BINARY_DIR}/sapi_base_force_cxx_linkage.cc"
+  "${SAPI_BINARY_DIR}/sapi_force_cxx_linkage.cc"
 )
 add_library(sapi::base ALIAS sapi_base)
 target_compile_features(sapi_base PUBLIC

cmake/SapiBuildDefs.cmake

@@ -111,10 +111,9 @@ function(add_sapi_library)
 
   # The sandboxed binary
   set(_sapi_bin "${_sapi_NAME}.bin")
-  set(_sapi_force_cxx_linkage
-    "${CMAKE_CURRENT_BINARY_DIR}/${_sapi_bin}_force_cxx_linkage.cc")
-  file(WRITE "${_sapi_force_cxx_linkage}" "")
-  add_executable("${_sapi_bin}" "${_sapi_force_cxx_linkage}")
+  add_executable("${_sapi_bin}"
+    "${SAPI_BINARY_DIR}/sapi_force_cxx_linkage.cc"
+  )
   target_link_libraries("${_sapi_bin}" PRIVATE
     -fuse-ld=gold
     -Wl,--whole-archive "${_sapi_LIBRARY}" -Wl,--no-whole-archive
@@ -192,10 +191,9 @@ function(add_sapi_library)
 
   # Library with the interface
   if(NOT _sapi_SOURCES)
-    set(_sapi_force_cxx_linkage
-      "${CMAKE_CURRENT_BINARY_DIR}/${_sapi_NAME}_force_cxx_linkage.cc")
-    file(WRITE "${_sapi_force_cxx_linkage}" "")
-    list(APPEND _sapi_SOURCES "${_sapi_force_cxx_linkage}")
+    list(APPEND _sapi_SOURCES
+      "${SAPI_BINARY_DIR}/sapi_force_cxx_linkage.cc"
+    )
   endif()
   add_library("${_sapi_NAME}" STATIC
     "${_sapi_gen_header}"

sandboxed_api/util/CMakeLists.txt

@@ -48,10 +48,9 @@ target_link_libraries(sapi_util_fileops PRIVATE
 )
 
 # sandboxed_api/util:flag
-file(WRITE "${SAPI_BINARY_DIR}/sapi_util_flags_force_cxx_linkage.cc" "")
 add_library(sapi_util_flags ${SAPI_LIB_TYPE}
   flag.h
-  "${SAPI_BINARY_DIR}/sapi_util_flags_force_cxx_linkage.cc"
+  "${SAPI_BINARY_DIR}/sapi_force_cxx_linkage.cc"
 )
 add_library(sapi::flags ALIAS sapi_util_flags)
 target_link_libraries(sapi_util_flags PUBLIC