build(wasi): compile pthreadShim in cxx

To avoid:

CMake Error: Error required internal CMake variable not set, cmake may not be built correctly.
  Missing variable is:
    CMAKE_C_COMPILE_OBJECT

Author: thewtex

src/docker/itk-wasm/ITKWebAssemblyInterface.cmake

@@ -64,7 +64,11 @@ 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/pthreadShim.c /ITKWebAssemblyInterface/src/initialization.cxx)
+      add_library(wasi-itk-extras STATIC
+        /ITKWebAssemblyInterface/src/exceptionShim.cxx
+        /ITKWebAssemblyInterface/src/cxaThreadAtExitShim.cxx
+        /ITKWebAssemblyInterface/src/pthreadShim.cxx
+        /ITKWebAssemblyInterface/src/initialization.cxx)
     endif()
     get_property(_is_imported TARGET ${wasm_target} PROPERTY IMPORTED)
     if (NOT ${_is_imported})

src/pthreadShim.c src/pthreadShim.cxxsrc/pthreadShim.c renamed to src/pthreadShim.cxx

@@ -20,6 +20,8 @@
 
 #include <pthread.h>
 
+extern "C" {
+
 int pthread_attr_setscope(pthread_attr_t *attr, int contentionscope)
 {}
 
@@ -30,3 +32,4 @@ int pthread_atfork(
 )
 {}
 
+} // end extern "C"