mod_backtrace on Wheezy on Raspberry Pi

Glibc backtrace() returns nothing without building the caller with -funwind-tables (or one of several related options); httpd, libapr, etc. all need similar for any real use of mod_backtrace. This is of course all related to the calling convention on ARM. (But casual use of gdb on plain builds of httpd didn’t show an impact. As usual, gdb is much more clever than backtrace().)