Fix "#if with no expression" errors in platform.h

[jmather-sesi]

Description

When building in debug mode, gcc rasied multiple "#if with no expression" errors with OSL_DEBUG.

Tests

Not applicable. Code compiles.

Checklist:

• I have read the contribution guidelines.
• I have updated the documentation, if applicable.
• I have ensured that the change is tested somewhere in the testsuite (adding new test cases if necessary).
• My code follows the prevailing code style of this project. If I haven't
  already run clang-format v17 before submitting, I definitely will look at
  the CI test that runs clang-format and fix anything that it highlights as
  being nonconforming.

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[lgritz]

I'm not sure why this is needed. The first mention of OSL_DEBUG in platform.h is at line 289:

#ifndef OSL_DEBUG
    #ifdef NDEBUG
        #define OSL_DEBUG 0
    #else
        #ifdef _DEBUG
            #define OSL_DEBUG _DEBUG
        #else
            #define OSL_DEBUG 1
        #endif
    #endif
#endif // OSL_DEBUG

That sure looks like it will always result in OSL_DEBUG being defined.

Additionally, in light of this, it's clear that "#ifdef OSL_DEBUG" is the wrong test, since sometimes it is defined and 0.

I feel like something else must be going on. What about that suspicious clause of #define OSL_DEBUG _DEBUG? Is it possible that _DEBUG is defined, but is empty?

[lgritz]

@jmather-sesi Any thoughts on my question?