Build: configure: drop useless cruft

• We don't follow the suggested structure
• No one's reading configure.ac for automake snippets
• We don't need ancient copy-and-pastes with no purpose for us
• We can assume size_t exists since we require C99
• We don't use SIZEOF macros, so don't bother defining them
• I have no idea how $Format:%h$ (added in 87629d537bd) was supposed to work, but nothing I know of uses that syntax