m4: stylistic-only: hoist AC_SUBST to be adjacent to initialization

m4: stylistic-only: hoist AC_SUBST to be adjacent to initialization

Declare a variable like LIB_CLOCK_GETTIME to be AC_SUBSTituted
right after its initialization, rather than farther down.
Keeping these in close proximity makes it easier to ensure
that each such variable is initialized.  E.g.,

    LIB_CLOCK_GETTIME=
    AC_SUBST([LIB_CLOCK_GETTIME])

This change also increments these serial numbers.
* m4/clock_time.m4 (gl_CLOCK_TIME): Hoist AC_SUBST use.
* m4/euidaccess.m4 (gl_PREREQ_EUIDACCESS): Likewise.
* m4/nanosleep.m4 (gl_FUNC_NANOSLEEP): Likewise.