Set build type to RelWithDebInfo

**Set build type to RelWithDebInfo**
This leads to build errors, as variables that are only used in asserts
get optimized out and because of -Werror being set (unused variables).

Fixed those warnings by using the following for `assert` (in `include/debug.h`):
```c
#ifdef LMMS_DEBUG
	#include <assert.h>
#else
	#define assert(x) ((void)(x))
#endif
```
Also fixed some other warnings, all happening in `plugins/midi_import/portsmf`.
Although it builds on my system, there may still be other warnings that need to be fixed, the Travis build will show that.