[7.4.0]

Bump version to 7.4.0; change policy regarding version numbers

* ChangeLog: Update.
* README.md: Bump version.
* configure.ac (AC_INIT): Likewise.
* src/atomic_ops/ao_version.h (AO_VERSION_MINOR): Likewise.
* src/atomic_ops/ao_version.h (AO_VERSION_ALPHA): Replace to
AO_VERSION_MICRO; add comment.