make swift fails because result of version.h.in is not found

Sources/libMultiMarkdown/mmd.c:75:10: fatal error: 'version.h' file not found

Indeed, it is not there hard-coded but depends on a template file which is compiled to `${PROJECT_BINARY_DIR}`, it seems, but despite the `include_directories(${PROJECT_BINARY_DIR})` directive cannot be found.

Will try to fix that, if I find out how cmake works.

```
ProductName: Mac OS X
ProductVersion: 10.13
BuildVersion: 17A365
---
Xcode 9.0
Build version 9A235
```