list_atomic: Add makefile rule to test list_atomic.template syntax

list_atomic: Add makefile rule to test list_atomic.template syntax

* tests/Makefile.am (BUILT_SOURCES, CLEANFILES): Add "list_atomic.o".
* tests/Makefile.am (list_atomic.o): New rule (to verify list_atomic.c
is valid C code).
* tests/list_atomic.template (list_atomicXX): Use "volatile" for "val"
local variable.
* tests/list_atomic.template (list_atomicXX): Define "oldval", "newval",
"ts" and "incr" local variables only if used; cast string literals to
void (to prevent compiler warnings).
* tests/list_atomic.template (list_atomicXX): Use AO_t type for "incr"
local variable instead of C long type.
* tests/list_atomic.template (list_atomicXX): Define "oldval",
"newval", "incr" as statitc local variables, and remove explicit
initialization to zero (to void problems with initialization of these
variables for AO_double_t case if the latter is a struct).