* configure.ac: Add -Wno-nullability-extension -Wno-nullability-completeness to WARN_CFLAGS
* include/wget/wget.h: Add CLANG_VERSION_AT_LEAST,
  fix attribute defines for clang,
  make use of attributes by using static inline functions for wget allocation,
  define NULLABLE for clang
* libwget/xalloc.c: Rename wget allocation function pointers
* src/options.c: Likewise
* unit-tests/test.c: Likewise