[PATCH] [EXPERIMENTAL] autocrap autohardening

[Nils Goroll]

brief summary of a #varnish-hacking discussion from my understanding (please
correct or comment if you disagree):

* we do want hardening options by default
* The patch needs to be tested against gcc and llvm/clang
* phk would like the compiler flag tests be implemented using the methodology
  from Makefile.phk / config.phk in favor of wrap-compiler-for-flag-check
* look after vcc cflags

related change:

* we want three basic compiler-settings
  - developer
  - production
  - performance
  in the order of decreasing safety settings. People should be discouraged
  from using "performance" unless absolutely necessary