Fix template-based headers regeneration order in src/Makefile

* src/atomic_ops/Makefile.am (BUILT_SOURCES): Define to force
regeneration of generalize-small.h (if the corresponding
source template is changed) before compilation of .c files (that use
atomic_ops.h).

Conflicts:

    src/Makefile.am
Regenerate Makefile.in; update ChangeLog