Enable DJGPP/MS-DOS build of jasper

Changing line https://github.com/jasper-software/jasper/blob/master/src/libjasper/base/jas_malloc.c#L651 from
```
#elif defined(__unix__) && (!defined(__linux__) && !defined(__APPLE__))
```
to
```
#elif defined(__unix__) && (!defined(__linux__) && !defined(__APPLE__)&& !defined(__DJGPP__))
```
allows Jasper to be cross compiled for MS-DOS using DJGPP
