Rework the test/releasetest.tcl script so that it uses the autoconf makefile
instead of the test/releasetest.mk makefile.  Also add options like --dryrun
and --buildonly.  Omit the --makefile option and replace it with --srcdir
with the default computed relative to the releasetest.tcl script itself.
Add the "checksymbols" target to Makefile.in
Fix a potential segfault following OOM error in the test harness.  The
SQLite core itself is not at fault.
Draft of changes necessary to make releasetest work on Windows via MinGW.
Add the threadtest target to Makefile.in.  Add --enable-load-extension to
the configure issued by releasetest.tcl.
In the releasetest.tcl script, show the test target for each configuration
that is run.  And show the time in HH:MM:SS.
Permit sqlite3_shutdown() to be called with OMIT_WSD enabled and without having called sqlite3_initialize() first.
Fix typo in library object file name.
Remove 'threadtest' as a target on Windows, due to lack of pthreads.  Also, change 'fulltest' to 'fulltestonly'.
Merge the new and improved releasetest.tcl script into trunk.  Add a
"make releasetest" target to the autoconf makefile.