build-aux/link-warning.h

/* GL_LINK_WARNING("literal string") arranges to emit the literal string as
   a linker warning on most glibc systems.
   We use a linker warning rather than a preprocessor warning, because
   #warning cannot be used inside macros.  */
#ifndef GL_LINK_WARNING
  /* This works on platforms with GNU ld and ELF object format.
     Testing __GLIBC__ is sufficient for asserting that GNU ld is in use.
     Testing __ELF__ guarantees the ELF object format.
     Testing __GNUC__ is necessary for the compound expression syntax.  */
# if defined __GLIBC__ && defined __ELF__ && defined __GNUC__
#  define GL_LINK_WARNING(message) \
     GL_LINK_WARNING1 (__FILE__, __LINE__, message)
#  define GL_LINK_WARNING1(file, line, message) \
     GL_LINK_WARNING2 (file, line, message)  /* macroexpand file and line */
#  define GL_LINK_WARNING2(file, line, message) \
     GL_LINK_WARNING3 (file ":" #line ": warning: " message)
#  define GL_LINK_WARNING3(message) \
     ({ static const char warning[sizeof (message)]		\
          __attribute__ ((__unused__,				\
                          __section__ (".gnu.warning"),		\
                          __aligned__ (1)))			\
          = message "\n";					\
        (void)0;						\
     })
# else
#  define GL_LINK_WARNING(message) ((void) 0)
# endif
#endif