Stop setting an empty AM_RUNTESTFLAGS
Stop looking for expect and runtest above top_builddir

Users wishing to test hand-compiled versions of expect and runtest can
easily enough put them in their path or set EXPECT and RUNTEST
themselves.
.travis.yml: Simplify the test matrix

Rather than creating a 2x2 matrix of compilers and environment then
excluding one, just add the one additional 32-bit build.
Always set CC_FOR_TARGET for dejagnu, to make the testsuite respect $CC

This fixes cross-compilation and compilation with CC="gcc -m32".
WIP: Test on Windows using wine