Bug 1317642 Disable WeakPtr thread assertions on MinGW r=froydnj

MinGW has two threading models: win32 API based, which disables std::thread,
and POSIX based which enables it but requires an emulation library (winpthreads).

Rather than attempting to switch to pthread emulation at this point, we are
disabling the std::thread based assertion checking for WeakPtr on MinGW.

MozReview-Commit-ID: BmHo70n6AuK

--HG--
extra : rebase_source : 08495775b2925a797c8098216911d30c0b79ef3d

mfbt/WeakPtr.h

@@ -88,8 +88,19 @@
 // its dereference, and destruction of the real object must all happen
 // on a single thread.  The following macros implement assertions for
 // checking these conditions.
+//
+// We disable this on MinGW. MinGW has two threading models: win32
+// API based, which disables std::thread; and POSIX based which
+// enables it but requires an emulation library (winpthreads).
+// Rather than attempting to switch to pthread emulation at this point,
+// we are disabling the std::thread based assertion checking.
+//
+// In the future, to enable it we could
+// a. have libgcc/stdc++ support win32 threads natively
+// b. switch to POSIX-based threading in MinGW with pthread emulation
+// c. refactor it to not use std::thread
 
-#if defined(DEBUG) || (defined(NIGHTLY_BUILD) && !defined(MOZ_PROFILING))
+#if !defined(__MINGW32__) && (defined(DEBUG) || (defined(NIGHTLY_BUILD) && !defined(MOZ_PROFILING)))
 
 #include <thread>
 #define MOZ_WEAKPTR_DECLARE_THREAD_SAFETY_CHECK \