Define internal M_INLINE:

it is used to control the inline semantics.
By combining it with optimize option "-fno-inline"
for the internal M*LIB function (using _Pragma optimize)
we can control easily if we want to use an inline version
or call of an external function.

Define new user API:
* M_USE_EXTERN_DECL : define it before including any MLIB header to requests to call the M*LIB functions and don't inline then.
* M_USE_EXTERN_DEF  : define it once in a translation unit to define the M*LIB functions. This translation unit shall contain all templated macro too.
If you use one option, you need to use the other one too for another translation unit.

m-atomic.h

@@ -226,7 +226,7 @@ typedef _Atomic(uintmax_t)          atomic_uintmax_t;
 #endif
 
 /* (INTERNAL) Unlock the mutex and return the given value */
-static inline long long atomic_fetch_unlock (m_mutex_t *lock, long long val)
+M_INLINE long long atomic_fetch_unlock (m_mutex_t *lock, long long val)
 {
   m_mutex_unlock (*lock);
   return val;