isnan*: Separate the AC_LIBOBJ invocations.

isnan*: Separate the AC_LIBOBJ invocations.

* m4/isnanf.m4 (gl_PREREQ_ISNANF): Renamed from gl_BUILD_ISNANF. Remove
AC_LIBOBJ invocation.
(gl_FUNC_ISNANF, gl_FUNC_ISNANF_NO_LIBM): Update. Invoke AC_LIBOBJ
here.
* m4/isnand.m4 (gl_PREREQ_ISNAND): Renamed from gl_BUILD_ISNAND. Remove
AC_LIBOBJ invocation.
(gl_FUNC_ISNAND, gl_FUNC_ISNAND_NO_LIBM): Update. Invoke AC_LIBOBJ
here.
* m4/isnanl.m4 (gl_PREREQ_ISNANL): Renamed from gl_BUILD_ISNANL. Remove
AC_LIBOBJ invocation.
(gl_FUNC_ISNANL, gl_FUNC_ISNANL_NO_LIBM): Update. Invoke AC_LIBOBJ
here.
* m4/isnan.m4 (gl_ISNAN): Update. Invoke AC_LIBOBJ here.