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.
