The build location of the resulting libatomic_ops.a was changed in v7.4.0
after converting libatomic_ops to use libtool.  The fix is to use library
from the install path, this allows building with both old and new versions.

Initially reported here:
https://mailman.nginx.org/pipermail/nginx/2018-April/056054.html

Using "long *" instead of "AO_t *" leads either to -Wincompatible-pointer-types
or -Wpointer-sign warnings, depending on whether long and size_t are compatible
types (e.g., ILP32 versus LP64 data models).  Notably, -Wpointer-sign warnings
are enabled by default in Clang only, and -Wincompatible-pointer-types is an
error starting from GCC 14.

Signed-off-by: Edgar Bonet <bonet@grenoble.cnrs.fr>