`interprocess::ipcdetail::intermodule_singleton` not working on MinGW

[joaquintides]

As shown in this CI job from Boost.Flyweight:

2024-09-14T10:12:05.5807995Z gcc.compile.c++ bin.v2\libs\flyweight\test\test_intermod_holder.test\gcc-8\debug\x86_64\cxxstd-14-iso\threadapi-win32\threading-multi\visibility-hidden\test_intermod_holder.o
2024-09-14T10:12:05.8093853Z gcc.compile.c++ bin.v2\libs\flyweight\test\test_no_locking.test\gcc-8\debug\x86_64\cxxstd-14-iso\threadapi-win32\threading-multi\visibility-hidden\test_no_locking_main.o
2024-09-14T10:12:08.2337843Z gcc.link bin.v2\libs\flyweight\test\test_intermod_holder.test\gcc-8\debug\x86_64\cxxstd-14-iso\threadapi-win32\threading-multi\visibility-hidden\test_intermod_holder.exe
2024-09-14T10:12:08.2709878Z testing.capture-output bin.v2\libs\flyweight\test\test_intermod_holder.test\gcc-8\debug\x86_64\cxxstd-14-iso\threadapi-win32\threading-multi\visibility-hidden\test_intermod_holder.run
2024-09-14T10:12:08.2711770Z ====== BEGIN OUTPUT ======
2024-09-14T10:12:08.2720797Z libs/flyweight/test/test_intermod_holder.cpp(33): test 'str==intermodule_flyweight_string("boost")' failed in function 'void test_intermodule_holder()'
2024-09-14T10:12:08.2734316Z 1 error detected.

The failing test seems to indicate that the singleton is created anew in each module instead of shared among them. As you probably remember, boost::flyweight::intermodule_holder is a trivial wrapper over interprocess::ipcdetail::intermodule_singleton:

https://github.com/boostorg/flyweight/blob/develop/include/boost/flyweight/intermodule_holder.hpp