Avoid unnecessary regeneration of openslide-tables.c

by building make-tables via recursive make invocation.  Fixes distcheck
error.
Drop cross-compile exclusion for building openslide-tables.c

Since we're more conservative about regenerating openslide-tables.c,
this should be safe as long as make-tables.c is not modified.  It also
fixes building from Git with MinGW on Cygwin.
Makefile.am: Remember to clean make-tables