CPython and Windows CRT debug builds use different values for uninit,
freed, and padding bytes. Make ``MI_DEBUG_*`` constants conditional to
allow embedders to override the constants.
Windows dbgheap:
```
_bNoMansLandFill = 0xFD
_bDeadLandFill = 0xDD
_bCleanLandFill = 0xCD
```
Python memory debug
```
PYMEM_CLEANBYTE 0xCD
PYMEM_DEADBYTE 0xDD
PYMEM_FORBIDDENBYTE 0xFD
```
Signed-off-by: Christian Heimes <christian@python.org>
0353fc38dd