Fix AM_CONFIG_HEADER in configure for autoconf-2.69-1

* configure.ac (AM_CONFIG_HEADER): Replace obsolete macro with with
AC_CONFIG_HEADERS one.
Fix typo in comment of generalize-small.template
(Apply commit 22d7c97 from 'master' branch.)

* src/atomic_ops/generalize.h: Fix a typo in comment
("acquire").

Conflicts:

    src/atomic_ops/generalize-arithm.h
    src/atomic_ops/generalize-arithm.template
Fix ARMv7 LDREXD/STREXD double-wide operand specification (GCC/Clang)
(Apply commit db2eef2 from 'master' branch.)

* src/atomic_ops/sysdeps/gcc/arm.h
(AO_double_compare_and_swap): Specify that LDREXD and STREXD use 2
adjacent registers (thus preventing Clang3.3 from register allocation
failures leading to "registers may not be the same" or
"even register required" GAS errors).

Conflicts:

    src/atomic_ops/sysdeps/gcc/arm.h
Fix ARM double-CAS operands width (GCC/Clang)
(Apply commit a81940c from 'master' branch.)

* src/atomic_ops/sysdeps/gcc/arm.h (AO_double_compare_and_swap): Swap
assembly code operands ("new_val" and "addr") to prevent
Clang3.3 warning about operand truncation.

Conflicts:

    src/atomic_ops/sysdeps/gcc/arm.h
Fix: Do not use LDREXD/STREXD for Clang3.2/arm (and earlier)
(Apply commit 2daf685 from 'master' branch.)

* src/atomic_ops/sysdeps/gcc/arm.h
(AO_compare_double_and_swap_double): Do not define for
pre-Clang3.3 (since the latter does not allocate register pairs for
LDREXD/STREXD instructions properly); add comment.

Conflicts:

    src/atomic_ops/sysdeps/gcc/arm.h
Regenerate aclocal.m4; update ChangeLog