glibc needs _GNU_SOURCE defined to make IOV_MAX visible. Use AC_USE_SYSTEM_EXTENSIONS and make sure to include compat.h before everything else. Found by cage_, thanks!

using "> " as prefix for both first and continuation lines was a bit ambiguous.

p->buf and buf almost always overlap. This means that once we free(p->buf), buf becomes invalid. It probably never crashed on memcpy since because the buffer is almost always quite small.

found by cage_: thanks!