Disable Zstandard for now.

Because of bug #90 the current build process will be broken for people
who have Zstandard headers available on their host system. For people
who do not have the headers available the build system will silently
ignore the `--enable-zstd` and build Tor without support for it.

See: https://github.com/n8fr8/orbot/issues/90

external/Makefile

@@ -208,7 +208,7 @@ tor/Makefile: tor/configure
 				--enable-static-libevent --with-libevent-dir=$(EXTERNAL_ROOT) \
 				--enable-static-openssl --with-openssl-dir=$(EXTERNAL_ROOT) \
 				--disable-linker-hardening --disable-gcc-hardening --disable-tool-name-check \
-				--disable-lzma --enable-zstd
+				--disable-lzma --disable-zstd
 
 tor-build-stamp: tor/Makefile
 	$(MAKE) -C tor all-am