Small fixes

Two small fixes and one new feature: 
* I could not convince autogen.sh to pass --prefix=/usr/local to configure.sh, which is important to me
* the premature termination of optimisation with 'Finished solve_cw after 1 steps and 0 CG iters' was reported about a year ago; until the newly written code in 1.2.1 works, I suggest to fall back to the norm2() function from MEEP 1.1.1
* band-src-time uses the sine-integral time function to provide well-defined rectangular spectrum (see http://fzu.cz/~dominecf/misc/meep/index.html#bandsource for examples)