Windows build with MinGW fails

Trying to build with MinGW (GNU CC) on Windows to get uint128 support I get this error

```
[ 13%] Built target leak_check_disable
[ 13%] Building CXX object absl/time/CMakeFiles/time_zone.dir/internal/cctz/src/time_zone_posix.cc.obj
C:\.hunter\_Base\6c9b2bc\156a9f8\fec2e58\Build\abseil\Source\absl\time\internal\cctz\src\time_zone_impl.cc:40:6: error: 'mutex' in namespace 'std' does not name a type
40 | std::mutex& TimeZoneMutex() {
| ^~~~~
C:\.hunter\_Base\6c9b2bc\156a9f8\fec2e58\Build\abseil\Source\absl\time\internal\cctz\src\time_zone_impl.cc:26:1: note: 'std::mutex' is defined in header ''; did you forget to '#include '?
25 | #include "time_zone_fixed.h"
+++ |+#include
26 |
C:\.hunter\_Base\6c9b2bc\156a9f8\fec2e58\Build\abseil\Source\absl\time\internal\cctz\src\time_zone_impl.cc: In static member function 'static bool absl::lts_2020_09_23::time_internal::cctz::time_zone::Impl::LoadTimeZone(const string&, absl::lts_2020_09_23::time_internal::cctz::time_zone*)':
C:\.hunter\_Base\6c9b2bc\156a9f8\fec2e58\Build\abseil\Source\absl\time\internal\cctz\src\time_zone_impl.cc:63:26: error: 'mutex' is not a member of 'std'
63 | std::lock_guard lock(TimeZoneMutex());
| ^~~~~
C:\.hunter\_Base\6c9b2bc\156a9f8\fec2e58\Build\abseil\Source\absl\time\internal\cctz\src\time_zone_impl.cc:63:26: note: 'std::mutex' is defined in header ''; did you forget to '#include '?
C:\.hunter\_Base\6c9b2bc\156a9f8\fec2e58\Build\abseil\Source\absl\time\internal\cctz\src\time_zone_impl.cc:63:31: error: template argument 1 is invalid
63 | std::lock_guard lock(TimeZoneMutex());
| ^
C:\.hunter\_Base\6c9b2bc\156a9f8\fec2e58\Build\abseil\Source\absl\time\internal\cctz\src\time_zone_impl.cc:63:38: error: 'TimeZoneMutex' was not declared in this scope
63 | std::lock_guard lock(TimeZoneMutex());
| ^~~~~~~~~~~~~
C:\.hunter\_Base\6c9b2bc\156a9f8\fec2e58\Build\abseil\Source\absl\time\internal\cctz\src\time_zone_impl.cc:63:33: warning: unused variable 'lock' [-Wunused-variable]
63 | std::lock_guard lock(TimeZoneMutex());
| ^~~~
C:\.hunter\_Base\6c9b2bc\156a9f8\fec2e58\Build\abseil\Source\absl\time\internal\cctz\src\time_zone_impl.cc:77:24: error: 'mutex' is not a member of 'std'
77 | std::lock_guard lock(TimeZoneMutex());
| ^~~~~
C:\.hunter\_Base\6c9b2bc\156a9f8\fec2e58\Build\abseil\Source\absl\time\internal\cctz\src\time_zone_impl.cc:77:24: note: 'std::mutex' is defined in header ''; did you forget to '#include '?
C:\.hunter\_Base\6c9b2bc\156a9f8\fec2e58\Build\abseil\Source\absl\time\internal\cctz\src\time_zone_impl.cc:77:29: error: template argument 1 is invalid
77 | std::lock_guard lock(TimeZoneMutex());
| ^
C:\.hunter\_Base\6c9b2bc\156a9f8\fec2e58\Build\abseil\Source\absl\time\internal\cctz\src\time_zone_impl.cc:77:36: error: 'TimeZoneMutex' was not declared in this scope
77 | std::lock_guard lock(TimeZoneMutex());
| ^~~~~~~~~~~~~
C:\.hunter\_Base\6c9b2bc\156a9f8\fec2e58\Build\abseil\Source\absl\time\internal\cctz\src\time_zone_impl.cc:77:31: warning: unused variable 'lock' [-Wunused-variable]
77 | std::lock_guard lock(TimeZoneMutex());
| ^~~~
C:\.hunter\_Base\6c9b2bc\156a9f8\fec2e58\Build\abseil\Source\absl\time\internal\cctz\src\time_zone_impl.cc: In static member function 'static void absl::lts_2020_09_23::time_internal::cctz::time_zone::Impl::ClearTimeZoneMapTestOnly()':
C:\.hunter\_Base\6c9b2bc\156a9f8\fec2e58\Build\abseil\Source\absl\time\internal\cctz\src\time_zone_impl.cc:88:24: error: 'mutex' is not a member of 'std'
88 | std::lock_guard lock(TimeZoneMutex());
| ^~~~~
C:\.hunter\_Base\6c9b2bc\156a9f8\fec2e58\Build\abseil\Source\absl\time\internal\cctz\src\time_zone_impl.cc:88:24: note: 'std::mutex' is defined in header ''; did you forget to '#include '?
C:\.hunter\_Base\6c9b2bc\156a9f8\fec2e58\Build\abseil\Source\absl\time\internal\cctz\src\time_zone_impl.cc:88:29: error: template argument 1 is invalid
88 | std::lock_guard lock(TimeZoneMutex());
| ^
C:\.hunter\_Base\6c9b2bc\156a9f8\fec2e58\Build\abseil\Source\absl\time\internal\cctz\src\time_zone_impl.cc:88:36: error: 'TimeZoneMutex' was not declared in this scope
88 | std::lock_guard lock(TimeZoneMutex());
| ^~~~~~~~~~~~~
C:\.hunter\_Base\6c9b2bc\156a9f8\fec2e58\Build\abseil\Source\absl\time\internal\cctz\src\time_zone_impl.cc:88:31: warning: unused variable 'lock' [-Wunused-variable]
88 | std::lock_guard lock(TimeZoneMutex());
| ^~~~
make[5]: *** [absl\time\CMakeFiles\time_zone.dir\build.make:121: absl/time/CMakeFiles/time_zone.dir/internal/cctz/src/time_zone_impl.cc.obj] Error 1
make[5]: *** Waiting for unfinished jobs....
```
See also https://github.com/abseil/abseil-cpp/issues/12