illumos needs `alloca.h` to get `alloca()`

[mtelka]

To compile successfully src/apputils/argparse.cpp on illumos one needs to add the alloca.h include to the file.

See https://github.com/OpenIndiana/oi-userland/blob/e4f5cba886591d62fd365f7a941730796d2d5e46/components/library/opencolorio/patches/01-argparse.cpp.patch for reference.