cmake: simplify by not using FetchContent

CMakeLists.txt

@@ -18,7 +18,9 @@ message(STATUS "p4est ${PROJECT_VERSION} "
 
 # --- external libs
 
-include(cmake/sc.cmake)
+include(cmake/GitSubmodule.cmake)
+git_submodule("${PROJECT_SOURCE_DIR}/sc")
+add_subdirectory(sc)
 
 # --- configure p4est
 
@@ -37,7 +39,7 @@ target_include_directories(p4est PUBLIC
   $<INSTALL_INTERFACE:include>)
 target_link_libraries(p4est PUBLIC SC::SC $<$<BOOL:${P4EST_HAVE_WINSOCK2_H}>:${WINSOCK_LIBRARIES}>)
 
-# imported target, for use from FetchContent
+# imported target, for use from parent projects
 add_library(P4EST::P4EST INTERFACE IMPORTED GLOBAL)
 target_link_libraries(P4EST::P4EST INTERFACE p4est)
 

cmake/GitSubmodule.cmake

@@ -1,17 +1,27 @@
-function(git_submodule dir)
+# else it's an offline archive
+if(IS_DIRECTORY ${PROJECT_SOURCE_DIR}/.git)
+  find_package(Git REQUIRED)
+endif()
+
+function(git_submodule submod_dir)
 # get/update Git submodule directory to CMake, assuming the
 # Git submodule directory is a CMake project.
 
-find_package(Git REQUIRED)
+if(NOT IS_DIRECTORY ${PROJECT_SOURCE_DIR}/.git)
+  message(DEBUG "${PROJECT_SOURCE_DIR} is not a Git repository, skipping submodule ${submod_dir}")
+  return()
+endif()
+
+if(EXISTS ${submod_dir}/CMakeLists.txt)
+  return()
+endif()
 
-if(NOT EXISTS ${dir}/CMakeLists.txt)
-  execute_process(COMMAND ${GIT_EXECUTABLE} submodule update --init --recursive -- ${dir}
-    WORKING_DIRECTORY ${PROJECT_SOURCE_DIR}
-    RESULT_VARIABLE _err)
+execute_process(COMMAND ${GIT_EXECUTABLE} submodule update --init --recursive -- ${submod_dir}
+WORKING_DIRECTORY ${PROJECT_SOURCE_DIR}
+RESULT_VARIABLE err)
 
-  if(NOT _err EQUAL 0)
-    message(SEND_ERROR "Could not retrieve Git submodule ${dir}.")
-  endif()
+if(NOT err EQUAL 0)
+  message(FATAL_ERROR "${submod_dir} Git submodule failed to retrieve.")
 endif()
 
-endfunction(git_submodule)
+endfunction()

cmake/config.cmake

@@ -113,7 +113,7 @@ set(P4EST_HAVE_FSYNC ${SC_HAVE_FSYNC} CACHE BOOL "platform has fsync")
 set(P4EST_HAVE_GETOPT_H ${SC_HAVE_GETOPT_H} CACHE BOOL "platform has getopt.h")
 
 if(P4EST_HAVE_ZLIB)
-  # ZLIB::ZLIB would be defined in sc/cmake/zlib.cmake IMPORTED INTERFACE GLOBAL via libsc FetchContent
+  # ZLIB::ZLIB would be defined in sc/cmake/zlib.cmake IMPORTED INTERFACE GLOBAL via libsc
   if(NOT TARGET ZLIB::ZLIB)
     # libsc didn't build Zlib, so must find an existing Zlib
     find_package(ZLIB REQUIRED)