Adds a new method to the compiler object, has_define.
This makes it possible to check if a preprocessor macro/define
is set or not.
This is especially helpful if the define in question is empty,
for example:
#define MESON_EMPTY_DEFINE
This would yield the same results as a missing define with
the existing get_define method, as it would return an empty
string for both cases. Therefore this additional method is
needed.
Marvin Scholz