Use __func__ rather than __FUNCTION__ for C99 onwards

[nigels-com]

In make debug=1 mode for gcc 13.3.0, lots of warnings about the use of FUNCTION

debug.h:46:65: warning: ISO C does not support ‘__FUNCTION__’ predefined identifier [-Wpedantic]
   46 |                 printf("%-12s +%-4u %-20s : ",__FILE__,__LINE__,__FUNCTION__); \
      |                                                                 ^~~~~~~~~~~~

For C99 onwards, use func instead.

[nigels-com]

Updated to use __func__ for clang builds too.