Move SHAVE_INIT behind all other checks, in particular AG_GST_CHECK_CHECKS.
This should fix problems with header checking and checking for localtime_r,
which causes compilation errors with clean checkouts where common/shave has
not been created yet when those checks are run. It seems like SHAVE_INIT
changes the environment so that checks depending on a compiler need shave
to exist at that point, which will fail if AC_OUTPUT hasn't created it yet.

Fixes #605930.