build(wasi): add pthread_atfork, pthread_attr_setscope shims

For:

  wasm-ld: error: lto.tmp: undefined symbol: pthread_atfork
  wasm-ld: error: lto.tmp: undefined symbol: pthread_attr_setscope

src/docker/itk-wasm/ITKWebAssemblyInterface.cmake

@@ -64,7 +64,7 @@ function(add_executable target)
     # WASI
     set_property(TARGET ${wasm_target} PROPERTY SUFFIX ".wasi.wasm")
     if (NOT TARGET wasi-itk-extras AND DEFINED CMAKE_CXX_COMPILE_OBJECT)
-      add_library(wasi-itk-extras STATIC /ITKWebAssemblyInterface/src/exceptionShim.cxx /ITKWebAssemblyInterface/src/cxaThreadAtExitShim.cxx /ITKWebAssemblyInterface/src/initialization.cxx)
+      add_library(wasi-itk-extras STATIC /ITKWebAssemblyInterface/src/exceptionShim.cxx /ITKWebAssemblyInterface/src/cxaThreadAtExitShim.cxx /ITKWebAssemblyInterface/src/posixShim.c /ITKWebAssemblyInterface/src/initialization.cxx)
     endif()
     get_property(_is_imported TARGET ${wasm_target} PROPERTY IMPORTED)
     if (NOT ${_is_imported})

src/pthreadShim.c

@@ -0,0 +1,32 @@
+/*=========================================================================
+ *
+ *  Copyright NumFOCUS
+ *
+ *  Licensed under the Apache License, Version 2.0 (the "License");
+ *  you may not use this file except in compliance with the License.
+ *  You may obtain a copy of the License at
+ *
+ *         https://www.apache.org/licenses/LICENSE-2.0.txt
+ *
+ *  Unless required by applicable law or agreed to in writing, software
+ *  distributed under the License is distributed on an "AS IS" BASIS,
+ *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *  See the License for the specific language governing permissions and
+ *  limitations under the License.
+ *
+ *=========================================================================*/
+// Workaround for current lack of these functions in WASI. They are not
+// implemented, and WASI does not have POSIX processes.
+
+#include <pthread.h>
+
+int pthread_attr_setscope(pthread_attr_t *attr, int contentionscope)
+{}
+
+int pthread_atfork(
+    void (*prepare)(void),    // Called before fork
+    void (*parent)(void),     // Called in parent after fork
+    void (*child)(void)       // Called in child after fork
+)
+{}
+