b5bc5d69 breaks tons of old compilers, but configure does not reject them

b5bc5d69 introduced a reliance on force_align_arg_pointer for stack realignment, but does not impose any compiler checks in configure. As a result, many older (default on some distros!) compilers silently generate subtly very broken code, since configure will happily accept compilers that do not supports this now fundementally required attribute.

They will build, for example, with a ton of these warnings:

encoder/lookahead.c:91:1: warning: ‘force_align_arg_pointer’ attribute ignored [-Wattributes]

and generate simply broken code.

I suspect more users would be complaining if it were not for #8 (closed).