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

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.

Open src/apputils/argparse.cpp and inspect its existing includes and use of alloca(). Use the linked OpenIndiana patch as reference for the illumos-specific include, then verify that the file compiles successfully on illumos.

Written by the indexing model from the issue text.