defining the macro _GNU_SOURCE appears to create troubles in some cases. (#693)

* defining the macro _GNU_SOURCE appears to create troubles in some cases.

* definining _GNU_SOURCE can cause problems.

---------

Co-authored-by: Daniel Lemire <dlemire@lemire.me>

Author: lemire

include/roaring/portability.h

@@ -16,9 +16,10 @@
 #ifndef CROARING_INCLUDE_PORTABILITY_H_
 #define CROARING_INCLUDE_PORTABILITY_H_
 
-#ifndef _GNU_SOURCE
-#define _GNU_SOURCE 1
-#endif  // _GNU_SOURCE
+// Users who need _GNU_SOURCE should define it?
+// #ifndef _GNU_SOURCE
+// #define _GNU_SOURCE 1
+// #endif  // _GNU_SOURCE
 #ifndef __STDC_FORMAT_MACROS
 #define __STDC_FORMAT_MACROS 1
 #endif  // __STDC_FORMAT_MACROS