We shouldn't allow mutex lock/unlock to silently do nothing if NULL is
passed, as that is very likely a bug. Beyond that the Windows version
doesn't do this, so it's inconsistent as well.
This moves a lot of the preprocessor flags for what compiler/platform
we're using into a libqb-common.h header inside the libqb/include
folder. This gets included at the top of every libqb .cpp file, and is
intended to be fairly small, providing only necessary things like
_WIN32_WINNT (which needs to be defined before including <windows.h> or
friends).
This adds generic APIs to libqb for handling thread's, mutex's, and
condition variables. On Linux and OSX these are implemented via the ones
provided by pthreads. On Windows they're implemented via the ones
provided by the Win32 API.
For compiling, the code itself is not conditional, but the Makefile
includes logic to decide which implementation to pick.
Note that it would have been nice to simply use std::thread and friends
from C++11, however using them on MinGW appears to be a bit messy. Since
using the Windows ones directly isn't that hard this was an easy compromise.
