Failure to build debug version under Windows

[mrbean-bremen]

After the latest change (always link statically under Windows), debug builds of kiwisolver (and possibly other modules) fail with linker errors due to missing debug symbols:

variable.obj : error LNK2001: unresolved external symbol _invalid_parameter
libcpmtd.lib(locale.obj) : error LNK2001: unresolved external symbol _invalid_parameter
libcpmtd.lib(wlocale.obj) : error LNK2001: unresolved external symbol _invalid_parameter
libcpmtd.lib(xlocale.obj) : error LNK2001: unresolved external symbol _invalid_parameter
constraint.obj : error LNK2001: unresolved external symbol _invalid_parameter
expression.obj : error LNK2001: unresolved external symbol _invalid_parameter
solver.obj : error LNK2001: unresolved external symbol _invalid_parameter
term.obj : error LNK2001: unresolved external symbol _invalid_parameter
libcpmtd.lib(_tolower.obj) : error LNK2001: unresolved external symbol _calloc_dbg
libcpmtd.lib(locale.obj) : error LNK2001: unresolved external symbol _calloc_dbg
libcpmtd.lib(wlocale.obj) : error LNK2001: unresolved external symbol _calloc_dbg
libcpmtd.lib(xlocale.obj) : error LNK2001: unresolved external symbol _calloc_dbg
...

The reason can be seen in the warning:

cl : Command line warning D9025 : overriding '/MDd' with '/MT'

e.g. while overriding the flag the debug mode is ignored.

[MatthieuDartiailh]

How do you switch to debug build ? I am asking because limiting debug build to debug Python as done in #29 may still be surprising. I probably make sense to force debug in that case but we should be able to manually set it.

[mrbean-bremen]

Yes, I had also been thinking about that. The check for debug Python is actually taken from Pillow (who took it from elsewhere), and it matches our use case (at least under Windows, there release and debug libraries cannot be mixed), but I guess an option would make sense.
Do you have any preference on how to set this?

[MatthieuDartiailh]

(at least under Windows, there release and debug libraries cannot be mixed)

Do you have a reference about this ? The changes only affect windows so if it is true we can merge your PR.

[mrbean-bremen]

Hm, can't find a good reference right now, but it boils to the following (taken from SO):

Mixing debug and release code is bad practice. The problem is that the different versions can depend on different fundamental parts of the C++ runtime library, such as how memory is allocated, structures for things like iterators might be different, extra code could be generated to perform operations (e.g. checked iterators).

Basically, mixing debug and release runtime libraries at runtime can lead to various hard-to-find problems, mostly related to memory corruption, which I have seen multiple times.
At compile time, there may be linker errors (if using static libraries) due to different referenced symbols, like in the mentioned case. As far as I know, this is only a problem under Windows.