depends: allow for CONFIG_SITE to be used rather than stealing prefix
depends: allow for CONFIG_SITE to be used rather than stealing prefix

This does not break any existing prefix behavior, only makes new behavior work.

For example:
CONFIG_SITE=$PWD/depends/x86_64-pc-linux-gnu/share/config.site ./configure --prefix=/

marlengit/BitcoinUnlimited,Bitcoin-com/BUcash,Bitcoin-com/BUcash,cddjr/BitcoinUnlimited,arruah/ensocoin,Bitcoin-com/BUcash,arruah/ensocoin,marlengit/BitcoinUnlimited,Justaphf/BitcoinUnlimited,arruah/ensocoin,cddjr/BitcoinUnlimited,cddjr/BitcoinUnlimited,marlengit/BitcoinUnlimited,Bitcoin-com/BUcash,arruah/ensocoin,AllanDoensen/BitcoinUnlimited,AllanDoensen/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited,marcusdiaz/BitcoinUnlimited,Justaphf/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited,Justaphf/BitcoinUnlimited,arruah/ensocoin,arruah/ensocoin,marlengit/BitcoinUnlimited,marcusdiaz/BitcoinUnlimited,ftrader-bitcoinunlimited/hardfork_prototype_1_mvf-bu,BTCfork/hardfork_prototype_1_mvf-bu,cddjr/BitcoinUnlimited,BTCfork/hardfork_prototype_1_mvf-bu,AllanDoensen/BitcoinUnlimited,Justaphf/BitcoinUnlimited,krzysztofwos/BitcoinUnlimited,marlengit/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited,marcusdiaz/BitcoinUnlimited,ftrader-bitcoinunlimited/hardfork_prototype_1_mvf-bu,ftrader-bitcoinunlimited/hardfork_prototype_1_mvf-bu,AllanDoensen/BitcoinUnlimited,krzysztofwos/BitcoinUnlimited,BTCfork/hardfork_prototype_1_mvf-bu,BTCfork/hardfork_prototype_1_mvf-bu,marlengit/BitcoinUnlimited,BTCfork/hardfork_prototype_1_mvf-bu,marcusdiaz/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited,AllanDoensen/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited,AllanDoensen/BitcoinUnlimited,Bitcoin-com/BUcash,cddjr/BitcoinUnlimited,krzysztofwos/BitcoinUnlimited,Justaphf/BitcoinUnlimited,krzysztofwos/BitcoinUnlimited,BTCfork/hardfork_prototype_1_mvf-bu,cddjr/BitcoinUnlimited,marcusdiaz/BitcoinUnlimited,krzysztofwos/BitcoinUnlimited,krzysztofwos/BitcoinUnlimited,ftrader-bitcoinunlimited/hardfork_prototype_1_mvf-bu,ftrader-bitcoinunlimited/hardfork_prototype_1_mvf-bu,Justaphf/BitcoinUnlimited,Bitcoin-com/BUcash,marcusdiaz/BitcoinUnlimited,ftrader-bitcoinunlimited/hardfork_prototype_1_mvf-bu