The build has been split up into separate object files, and each object
file is recompiled if the relevant source/header file changes.

Previously, it would never recompile the library, regardless of whether
any files had changed, so running `make clean` was necessary every time.

Firstly, simdjson.cpp doesn't actually include simdjson.h, and
luasimdjson.cpp includes both luasimdjson.h and simdjson.h. Rather than
specifying the dependencies manually (where there is room for error)
it's better to let the compiler describe the file dependencies (by
generating .d files with -MMD and -MP), this avoids mistakes and doesn't
require manual intervention if the files ever change.