Enable ZSTD support properly for Android

In legacy/trac#28766 (moved) we tried to enable ZSTD support for Android but just specifying --enable-zstd when compiling tor and extracting the library is not enough:

WARNING: Unable to find libzstd, install pkg-config, and check the PKG_CONFIG_PATH environment variable, or set ZSTD_CFLAGS and ZSTD_LIBS