maint: correct many abuses of the word "which".

maint: correct many abuses of the word "which".

* HACKING, NEWS, TODO, bootstrap, bootstrap.conf,
build-aux/ltmain.in, cfg.mk, configure.ac, doc/libtool.texi,
doc/notes.texi, gl/build-aux/bootstrap.in,
gl/build-aux/extract-trace, gl/build-aux/funclib.sh,
gl/doc/bootstrap.texi, libltdl/libltdl/lt_system.h,
libltdl/loaders/loadlibrary.c, libltdl/lt__strl.c,
libltdl/lt_dlloader.c, libltdl/ltdl.c, libtoolize.in,
m4/libtool.m4, m4/ltdl.m4, m4/ltsugar.m4, m4/lt~obsolete.m4,
tests/exceptions.at, tests/need_lib_prefix.at, tests/static.at:
Use "which" correctly, or replace with "that", "where" or "what"
as appropriate.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>