Inline attribute should be configurable

[JohanEngelen]

The always inline attribute should be a macro such that it can be configured (if the compiler does not support the particular way of enforcing inlining). For example:

ctbignum/include/ctbignum/mult.hpp

Line 30 in d1b2659

[[gnu::always_inline]]

#define CBN_ALWAYS_INLINE [[gnu::always_inline] somewhere in a, e.g., ctbignum/config.hpp file.
Then later more fancy stuff can be added there for other compilers when needed.

[niekbouman]

added in #31