Fixed 'undef' warning with gcc and clang

[lelegard]

Fix for this warning:

srt/platform_sys.h:48:5: error: "__APPLE__" is not defined, evaluates to 0 [-Werror=undef]
   48 | #if __APPLE__
      |     ^~~~~~~~~

With clang, it is possible to use a pragma diagnostics before #include <srt/srt.h> to mute the warning.

With gcc, however, the situation is critical. Although the same pragma exists, it does nothing due to a known gcc bug since 2012, never fixed. Apparently, the bug appears too early in the lex analysis phase and #pragma are not yet parsed. It seems a bit difficult to solve and nobody wanted to fix it in 9 years.

See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53431

The problem is important in applications with a strict "no warning policy" (-Wall -Wextra -Werror). If a warning is impossible to mute, the compilation fails.

Note: please note that #if __APPLE__ was replaced with #if defined(__APPLE__) && __APPLE__. Please keep it like this and do not use #ifdef __APPLE__. Because of the gcc bug, in order to compile the application, I have found no other solution than this:

#if defined(__GNUC__) && !defined(__APPLE__)
#define __APPLE__ 0
#define ZERO__APPLE__ 1
#endif

#include <srt/srt.h>

#if defined(ZERO__APPLE__)
#undef __APPLE__
#undef ZERO__APPLE__
#endif

So, if you use #ifdef __APPLE__, it won't work as expected.

[maxsharabayko]

Hi @lelegard. Thank you for the fix!
I would suggest to add a dedicated comment, otherwise sooner or later it will be changed back to #if __APPLE__.

Also CMake defines DARWIN here. It is always defined with either 0 or 1. Probably makes sense to use it instead. But does not sound like something to check in v1.4.4 release. 🙂