Always set CC_FOR_TARGET for dejagnu, to make the testsuite respect $CC

This fixes cross-compilation and compilation with CC="gcc -m32".
.travis.yml: Make the build command more readable by splitting at &&

"script" can contain multiple commands to run in sequence.
Use the proper C++ compiler to run C++ tests

Running the C compiler with -shared-libgcc -lstdc++ does not work on
non-GCC compilers.
README: Update Windows example to set both CC and CXX
README: Note the testsuite changes to respect $CC and $CXX
WIP: Test on Windows using wine