Add BOOST_PP_BOOL_true and BOOST_PP_BOOL_false definitions

I suggest to define a new macro
```c++
#define BOOST_PP_BOOL_true 1
#define BOOST_PP_BOOL_false 0
```
for make that possible:
```c++
BOOST_PP_IF(BOOST_PP_BOOL(true/false), "YES", "NO")
// ^-- Expands into 1/0
```
It seems like simple behavior and I have no idea why its not added yet.