build: Abolish the `$e', `$o' and `$t' variables.

I don't think anything ever used `$o'.  Use of `$e' gets in the way of
Automake's magic handling of executable suffixes under Cygwin.  This is
especially acute when executables are listed as tests to be run, because
Autoconf is buggy and sometimes strips off the suffix and sometimes
doesn't.

So we write everything out longhand.  Sorry.