Use AM_CONDITIONAL and if/endif for AMDEP.

Use AM_CONDITIONAL and if/endif for AMDEP.
* m4/depend.m4 (AM_DEP_TRACK): Use AM_CONDITIONAL to define AMDEP.
* automake.in (&add_depend2): Transform %AMDEP% into `AMDEP' (leading
to a configure time if/endif), or to `FALSE' (static removal of
the code).
(&handle_dependencies): Adjust to use `AMDEP_TRUE'.
* depend2.am: Use if/endif.
Adjust the 101 tests that use dependencies so that they
properly invoke aclocal before automake.  They need to `see'
`AM_CONDITIONAL([AMDEP], ...)'.