This works for me:
./autogen.sh && ./configure --enable-static-tor --with-libevent-dir=$MINGW_PREFIX/lib --with-openssl-dir=$MINGW_PREFIX/lib --with-zlib-dir=$MINGW_PREFIX/lib --disable-unittests --disable-module-dirauth --disable-manpage --disable-html-manual --disable-asciidoc && make
You need to install it probably.
Execute pacman -Ss libevent and then select package depending on your compiler. For example, pacman -S mingw-w64-clang-x86_64-libevent for clang.