Refactored configure.ac, added pkg-config support
Added --with-pkgconfigdir
autotools: removed duplication in configure.ac
ChangeLog: set future version to 0.8.7
sha1: run-time check for endianness

WORDS_BIGENDIAN is never defined what breaks sha1 on big-endian
architectures. Instead, make run-time check.
sha1: compile-time endianness check for some compilers

Newer gcc and clang provide macro __BYTE_ORDER__.