Set Stk::showWarnings() via LMMS_DEBUG

[zonkmachine]

Fixup of #3172

[tresf]

@zonkmachine I was going to comment on this before merge...

We should really consider configuring this debug value into a header to avoid unnecessary pre-compiler tags all over our code.

Perhaps a nice home for it would be here: https://github.com/LMMS/lmms/blob/master/src/lmmsconfig.h.in

[tresf]

Sorry... bad example... LMMS_VERSION is a better example despite lmmsconfig.h.in is probably a more appropriate home. Note how the logic in the code can use it directly, rather than changing the precompile behavior. This means we could do something like: Stk::showWarnings(LMMS_DEBUG) instead. :)

Also, side-note... probably overkill to have both lmmsconfig.h.in as well as lmmsversion.h.in We should consider consolidating these. /offtopic.

[zonkmachine]

This means we could do something like: Stk::showWarnings(LMMS_DEBUG) instead. :)

OK, interesting so...

#cmakedefine LMMS_DEBUG 1
#ifndef LMMS_DEBUG
#define LMMS_DEBUG 0
#endif

...

Stk::showWarnings(LMMS_DEBUG);

[tresf]

@zonkmachine poorly documented as many of the cmake features, but I believe this will do the trick:

#cmakedefine01 LMMS_DEBUG

Per example @webkit/Source/cmakeconfig.h.cmake#L4, documented here: https://cmake.org/cmake/help/v3.2/command/configure_file.html?highlight=cmakedefine01

Yes, you read that right, #cmakedefine01 :)