pre gcc v5 support and allow building without dbengine

[ktsaou]

gcc versions prior to 5 do not allow initializing static members/variables with this expression:

#define NETDATA_SPINLOCK_INITIALIZER (SPINLOCK){ .locked = false, }

They throw a message that the above is not a constant initializer. But they accept the same without the cast:

#define NETDATA_SPINLOCK_INITIALIZER { .locked = false, }

This PR changes the code to allow compiling with gcc versions prior to 5.

Also, global statistics fail when dbengine is not enabled. This PR fixes that too.