Merge all Makefile.am in /src tree (to have less auto-generated files

Merge all Makefile.am in /src tree (to have less auto-generated files
in the project subdirectories).

* configure.ac (AC_CONFIG_FILES): Remove src/atomic_ops/Makefile,
src/atomic_ops/sysdeps/Makefile.
* src/Makefile.am (SUBDIRS): Remove.
* src/Makefile.am (EXTRA_DIST, privatedir, private_HEADERS): Add from
src/atomic_ops/Makefile.am and src/atomic_ops/sysdeps/Makefile.am.
* src/Makefile.am (generalize-small): Move from
src/atomic_ops/Makefile.am.
* src/atomic_ops/Makefile.am, src/atomic_ops/sysdeps/Makefile.am:
Remove.