Merge remote-tracking branch 'afbjorklund/everything'

Merge remote-tracking branch 'afbjorklund/everything'

* afbjorklund/everything:
  Make configure work with older clang versions
  Add option for enabling more compiler warnings
  Add configure support for detecting clang
  Avoid bad function cast by using temporary
  Proper use of const qualifier char* strings
  Removed locally shadowed global variables
  Don't use undefined macros, use ifdef instead
  Add noreturn to functions that don't return
  Add missing extern prototypes, or make static
  Don't use reserved id macros in the headers
  Only compile getopt_long.c when needed
  Address issues found by lgtm.com alerts