- Instead of forcefully adding
  -Wall
  -Wextra
  -Wno-unused
  -Wcast-qual
  -Wcast-align
  -Wbad-function-cast
  -Wmissing-declarations
  -Wmissing-prototypes
  -Wnested-externs
  -Wredundant-decls
  -Wshadow
  -Wwrite-strings
  -Wundef
  to CFLAGS, 'configure' now tests if the flags are understood by the compiler