We seem to be building ed25519-donna-64 even when we have no uint128_t

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I tried building Tor with tcc, the Tiny C Compiler, and although configure
succeeds the build dies at

In file included from src/ext/ed25519/donna/ed25519_tor.c:41:
In file included from src/ext/ed25519/donna/ed25519-donna.h:12:
src/ext/ed25519/donna/ed25519-donna-portable.h:87: error: #error Need a uint128_t implementation!
make[1]: *** [Makefile:15055: src/ext/ed25519/donna/libed25519_donna_a-ed25519_tor.o] Error 1

Given that a 128-bit integer type isn't required by any standard, C or POSIX,
this should be checked for by configure at least and perhaps mentioned in the
INSTALL file or build instructions.
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