Bump version to 7.4.0; change policy regarding version numbers

Bump version to 7.4.0; change policy regarding version numbers

* ChangeLog: Update.
* README.md: Bump version.
* configure.ac (AC_INIT): Likewise.
* include/gc_version.h (GC_TMP_VERSION_MINOR): Likewise.
* alloc.c (GC_version, GC_get_version): Replace GC_TMP_ALPHA_VERSION
with GC_VERSION_MICRO.
* doc/debugging.html: Remove comment about "alpha" versions.
* doc/overview.html: Likewise.
* include/gc.h (GC_get_version): Update comment.
* include/gc_version.h (GC_NOT_ALPHA): Remove.
* include/gc_version.h (GC_TMP_ALPHA_VERSION): Replace with
GC_TMP_VERSION_MICRO; add comment about policy.
* m4/gc_set_version.m4 (GC_ALPHA_VERSION): Replace with
GC_VERSION_MICRO; update comments; adjust version format test.
* tests/test.c (GC_ALPHA_VERSION): Remove.
* tools/add_gc_prefix.c (GC_ALPHA_VERSION): Likewise.
* tools/gcname.c (GC_ALPHA_VERSION): Likewise.
* tests/test.c (CHECH_GCLIB_VERSION): Replace GC_ALPHA_VERSION with
GC_VERSION_MICRO.
* tools/add_gc_prefix.c (main): Replace "gc%d.%d[alpha%d]" print
format with "gc-%d.%d.%d" one.
* tools/gcname.c (main): Likewise.
* windows-untested/gc.ver (_BETA, GC_VERSION_MICRO): Remove.