Eliminate GCC warning about uninitialized 'hhdr' in GC_allochblk_nth

* allchblk.c (GC_allochblk_nth): Replace "for" condition with
conditional "return" since otherwise GCC reports
"'hhdr' may be used uninitialized" warning in case of -O0 optimization
level; remove comment about bogus compiler warning; refine comment
for "thishdr" local variable.
Fix dyn_load.c compilation for Android 4.3

* dyn_load.c (struct link_map, r_debug): Define explicitly (matching
that of Android 4.2 Bionic) instead of include linker.h (only if
PLATFORM_ANDROID); remove workaround enabled by
BIONIC_ELFDATA_REDEF_BUG regarding ELF_DATA redefinition (as linker.h
is not included); update comment.
Update ChangeLog file
Fix a typo in ChangeLog
Workaround 'ELF_DATA/EM_ALPHA redefined' warning in Android link.h
(revert part of commit a6e4ee9)

* dyn_load.c: Include asm/elf.h and linux/elf-em.h explicitly, and
undefine ELF_DATA and EM_ALPHA (if PLATFORM_ANDROID and
BIONIC_ELFDATA_REDEF_BUG are defined) to avoid "macro redefined"
warnings reported for link.h of Android Bionic.
Start of port to QNX
(Apply commit 5643815)

* include/private/gcconfig.h (I386, OS_TYPE, SA_RESTART, HEURISTIC1,
etext, DATASTART, _end, DATAEND, mach_type_known): Define for QNX.
Update AUTHORS file
Enable prefetch operations by default (GCC 3.0+)

* include/private/gcconfig.h (PREFETCH): Update comment; remove FIXME.
* include/private/gcconfig.h (NO_PREFETCH_FOR_WRITE): Define if
USE_I686_PREFETCH defined but not FORCE_WRITE_PREFETCH.
* include/private/gcconfig.h (PREFETCH, PREFETCH_FOR_WRITE,
CLEAR_DOUBLE): Reformat code (for IA64).
* include/private/gcconfig.h (PREFETCH, PREFETCH_FOR_WRITE): Make
default definition to GCC (v3+) built-in operation for all platforms
(instead of only for Linux/x64) unless NO_PREFETCH (or
NO_PREFETCH_FOR_WRITE, respectively) defined.
Update TODO file (add 1 item about parallel marker)
Update ChangeLog file
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.
Bump libgc version

* README.md: Bump version to 7.5.0.
* configure.ac: Likewise.
* include/gc_version.h (GC_TMP_VERSION_MINOR): Likewise.
Fix ABORT message in GC_restart_handler

* pthread_stop_world.c (GC_restart_handler): Fix ABORT message.
Allow to force GC_dump_regularly set on at compilation

* doc/README.macros (GC_DUMP_REGULARLY): Document.
* misc.c (GC_dump_regularly): Turn on by default if GC_DUMP_REGULARLY
macro defined (but not NO_DEBUGGING) - useful for embedded environments.
* misc.c (GC_init): Skip "GC_DUMP_REGULARLY" environments variable check
if GC_dump_regularly already set by the corresponding macro (only if not
NO_DEBUGGING).
Fix 'Bad signal in suspend_handler' abort on FreeBSD-9.2

* pthread_stop_world.c (GC_suspend_handler_inner): If signal number
is zero then silently return (instead of ABORT) if FREEBSD (to
workaround a bug in FreeBSD 9.2-RELEASE regarding signals processing,
bug is observed only if THREAD_LOCAL_ALLOC defined).