fix: restore extern C++ and fix POSIX header collection for modules

Author: LSUDOKO

cmake/templates/hpx.ixx.in

@@ -18,13 +18,13 @@ module;
 // violations.
 #include <hpx/config/std_headers.hpp>
 
-#if !defined(__ANDROID__) && !defined(ANDROID) && !defined(__APPLE__)
-#include <link.h>
-#endif
-#include <dlfcn.h>
-
 export module HPX.@cap_libname@;
 
+// Make sure the exported symbols are name-mangled using standard C++ rules.
+// This is necessary as otherwise the symbols exported from the shared libraries
+// will be exported from the module using a differently encoded name.
+extern "C++" {
+
 #if defined(HPX_MSVC)
 // disable warning C5244: '#include <filename>' in the purview of module
 // 'HPX.@cap_libname@' appears erroneous.  Consider moving that directive before the
@@ -51,3 +51,5 @@ export module HPX.@cap_libname@;
 #if defined(HPX_MSVC)
 #pragma warning(pop)
 #endif
+
+}   // extern "C++"