initializing a static variable in C++11 using constexpr inside the class

I came across this thread in which one of the upvoted answers suggested using constexpr

Since C++11 it can be done inside a class with constexpr.

class stat {
    public:
        // init inside class
        static constexpr double inlineStaticVar = 22;
};

The variable can now be accessed with:

stat::inlineStaticVar

so in my case I have something like this

namespace LG_Wrapper
{
    template <LG_Thread Thread>
    class LG_Wrapper_API EffectApplication : public ktApplication
    {
    public:
    static EffectApplication<Thread>& GetInstance();
    protected:
        .....
        .....
        static boost::recursive_mutex mResource;
      }
}

so instead of attempting to initialize the variable outside the class I would like to do it inside the class like this

static constexpr boost::recursive_mutex mResource ;

However when I do this I get the following error

error: constexpr static data member 'mResource' must have an initializer|

Any suggestions ?

I am using Mingw GCC 64 bit.