Use stdint.h types for fixed-size integers

All supported compilers provide a stdint.h header, so we can use this
instead of relying on autotools/cmake checks. In theory this could break
the ABI due to C++ name mangling changes if dbus_int64_t changes between
`long long` and `long`. However, it appears the only affected platform is
macOS where that level of binary compatibility is not guaranteed.
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