Segfault on program exit (with minimal sample)

There seems to be an issue with current Nvidia and Intel platform (AMD does not seem to have this issue) when I compile Boost.Compute in a dynamic library; segfault is reported during some OpenCL resource cleanup while exiting. The interesting thing is if I statically link the same library this becomes a non-issue. **Update:** On MacOS it doesn't seem to have this issue

I am able to reproduce this issue with a few lines of code ([here](https://github.com/rosenrodt/bug_opencl_boost_compute)). The site raising the exception might be a bit different (at `program::~program()` or `command_queue::~command_queue()` or `context::~context()`) if I added or removed a few boost::compute APIs here and there.

Any thoughts on why this happens? I am hesitant to call this a Boost.Compute bug, but I have tried myself and I can't reproduce the problem in plain OpenCL API.

A few notes:
- The sample code uses exclusively default context and default command queue.
- Tested on Boost 1.68.0 and current master branch at `36c89134`
- OS: Win10

Full stack trace 1 (Nvidia platform)
```
ntdll.dll!00007ffc9a8ae771() Unknown
tmmon64.dll!00000000617a0f2b() Unknown
nvopencl64.dll!00007ffc22e84bac() Unknown
nvopencl64.dll!00007ffc22a49562() Unknown
nvopencl64.dll!00007ffc22a38ece() Unknown
nvopencl64.dll!00007ffc22a38329() Unknown
nvopencl64.dll!00007ffc22a2de83() Unknown
nvopencl64.dll!00007ffc22a2e365() Unknown
nvopencl64.dll!00007ffc22a37afa() Unknown
mylib.dll!boost::compute::context::~context() Line 162 C++
mylib.dll!`boost::compute::system::default_context'::`2'::`dynamic atexit destructor for 'default_context''() C++
ucrtbased.dll!00007ffc2fd30af7() Unknown
ucrtbased.dll!00007ffc2fd30545() Unknown
ucrtbased.dll!00007ffc2fd3064a() Unknown
ucrtbased.dll!00007ffc2fd30ca1() Unknown
mylib.dll!__scrt_dllmain_uninitialize_c() Line 392 C++
mylib.dll!dllmain_crt_process_detach(const bool) Line 109 C++
mylib.dll!dllmain_crt_dispatch(HINSTANCE__ * const, const unsigned long, void * const) Line 134 C++
mylib.dll!dllmain_dispatch(HINSTANCE__ * const, const unsigned long, void * const) Line 209 C++
mylib.dll!_DllMainCRTStartup(HINSTANCE__ * const, const unsigned long, void * const) Line 251 C++
ntdll.dll!00007ffc9a8e50e7() Unknown
ntdll.dll!00007ffc9a91068c() Unknown
ntdll.dll!00007ffc9a91053d() Unknown
tmmon64.dll!000000006170698c() Unknown
kernel32.dll!00007ffc9853d05a() Unknown
ucrtbased.dll!00007ffc2fd300da() Unknown
ucrtbased.dll!00007ffc2fd30099() Unknown
ucrtbased.dll!00007ffc2fd303d6() Unknown
myexe.exe!__scrt_common_main_seh() Line 275 C++
myexe.exe!__scrt_common_main() Line 309 C++
myexe.exe!mainCRTStartup() Line 17 C++
kernel32.dll!00007ffc98537e94() Unknown
ntdll.dll!00007ffc9a907ad1() Unknown
```

Full stack trace 2 (Nvidia platform)
```
ntdll.dll!00007ffc9a8ae771() Unknown
tmmon64.dll!00000000617a0f2b() Unknown
nvopencl64.dll!00007ffc22e84bac() Unknown
nvopencl64.dll!00007ffc22bbc79d() Unknown
nvopencl64.dll!00007ffc22bbc697() Unknown
nvopencl64.dll!00007ffc22a3c4a2() Unknown
nvopencl64.dll!00007ffc22a2de8d() Unknown
nvopencl64.dll!00007ffc22a2e365() Unknown
nvopencl64.dll!00007ffc22a3c2a8() Unknown
mylib.dll!boost::compute::command_queue::~command_queue() Line 219 C++
mylib.dll!`boost::compute::system::default_queue'::`2'::`dynamic atexit destructor for 'queue''() C++
ucrtbased.dll!00007ffc2fd30af7() Unknown
ucrtbased.dll!00007ffc2fd30545() Unknown
ucrtbased.dll!00007ffc2fd3064a() Unknown
ucrtbased.dll!00007ffc2fd30ca1() Unknown
mylib.dll!__scrt_dllmain_uninitialize_c() Line 392 C++
mylib.dll!dllmain_crt_process_detach(const bool) Line 109 C++
mylib.dll!dllmain_crt_dispatch(HINSTANCE__ * const, const unsigned long, void * const) Line 134 C++
mylib.dll!dllmain_dispatch(HINSTANCE__ * const, const unsigned long, void * const) Line 209 C++
mylib.dll!_DllMainCRTStartup(HINSTANCE__ * const, const unsigned long, void * const) Line 251 C++
ntdll.dll!00007ffc9a8e50e7() Unknown
ntdll.dll!00007ffc9a91068c() Unknown
ntdll.dll!00007ffc9a91053d() Unknown
tmmon64.dll!000000006170698c() Unknown
kernel32.dll!00007ffc9853d05a() Unknown
ucrtbased.dll!00007ffc2fd300da() Unknown
ucrtbased.dll!00007ffc2fd30099() Unknown
ucrtbased.dll!00007ffc2fd303d6() Unknown
myexe.exe!__scrt_common_main_seh() Line 275 C++
myexe.exe!__scrt_common_main() Line 309 C++
myexe.exe!mainCRTStartup() Line 17 C++
kernel32.dll!00007ffc98537e94() Unknown
ntdll.dll!00007ffc9a907ad1() Unknown
```

Full stack trace 3 (Intel platform)
```
ucrtbase.dll!00007ffc96b2f08e() Unknown
vcruntime140.dll!00007ffc87d94b16() Unknown
intelocl64.dll!00007ffc2a7625e0() Unknown
intelocl64.dll!00007ffc2a75618d() Unknown
mylib.dll!boost::compute::command_queue::~command_queue() Line 219 C++
mylib.dll!`boost::compute::system::default_queue'::`2'::`dynamic atexit destructor for 'queue''() C++
ucrtbased.dll!00007ffc2fd30af7() Unknown
ucrtbased.dll!00007ffc2fd30545() Unknown
ucrtbased.dll!00007ffc2fd3064a() Unknown
ucrtbased.dll!00007ffc2fd30ca1() Unknown
mylib.dll!__scrt_dllmain_uninitialize_c() Line 392 C++
mylib.dll!dllmain_crt_process_detach(const bool) Line 109 C++
mylib.dll!dllmain_crt_dispatch(HINSTANCE__ * const, const unsigned long, void * const) Line 134 C++
mylib.dll!dllmain_dispatch(HINSTANCE__ * const, const unsigned long, void * const) Line 209 C++
mylib.dll!_DllMainCRTStartup(HINSTANCE__ * const, const unsigned long, void * const) Line 251 C++
ntdll.dll!00007ffc9a8e50e7() Unknown
ntdll.dll!00007ffc9a91068c() Unknown
ntdll.dll!00007ffc9a91053d() Unknown
tmmon64.dll!000000006170698c() Unknown
kernel32.dll!00007ffc9853d05a() Unknown
ucrtbased.dll!00007ffc2fd300da() Unknown
ucrtbased.dll!00007ffc2fd30099() Unknown
ucrtbased.dll!00007ffc2fd303d6() Unknown
myexe.exe!__scrt_common_main_seh() Line 275 C++
myexe.exe!__scrt_common_main() Line 309 C++
myexe.exe!mainCRTStartup() Line 17 C++
kernel32.dll!00007ffc98537e94() Unknown
ntdll.dll!00007ffc9a907ad1() Unknown
```