Fix build for 64bit field under OSX
Fix build for 64bit field under OSX

- caused by https://github.com/bitcoin/secp256k1/commit/8881212ebc43e67052ec06dec8beb459769fbab7
- OSX's ar tool doesn't work for empty archives ("ar: no archive members specified")
- introduce COMMON_LIB variable; leave empty when not using asm

bitcoin-core/secp256k1,BTCfork/hardfork_prototype_1_mvf-bu,pmienk/secp256k1,jonasschnelli/secp256k1,libbitcoin/secp256k1,ChrisXin/secp256k1,digitalbitbox/secp256k1,chfast/secp256k1,chfast/secp256k1,ChrisXin/secp256k1,kleetus/secp256k1,BitcoinUnlimited/BitcoinUnlimited,rustyrussell/secp256k1,voisine/secp256k1,GSongHashrate/secp256k1,digitalbitbox/secp256k1,chfast/secp256k1,BTCfork/hardfork_prototype_1_mvf-bu,digitalbitbox/secp256k1,marcusdiaz/BitcoinUnlimited,BTCfork/hardfork_prototype_1_mvf-bu,kleetus/secp256k1,bitcoin-core/secp256k1,voisine/secp256k1,afk11/secp256k1,voisine/secp256k1,apoelstra/secp256k1,afk11/secp256k1,instagibbs/secp256k1,ftrader-bitcoinunlimited/hardfork_prototype_1_mvf-bu,Justaphf/BitcoinUnlimited,ElementsProject/secp256k1-zkp,krzysztofwos/BitcoinUnlimited,Bitcoin-com/BUcash,digitalbitbox/secp256k1,01BTC10/secp256k1,sipa/secp256k1,randy-waterhouse/secp256k1,instagibbs/secp256k1,BitcoinUnlimited/BitcoinUnlimited,BTCfork/hardfork_prototype_1_mvf-bu,pmienk/secp256k1,libmetrocoin/secp256k1,randy-waterhouse/secp256k1,cddjr/BitcoinUnlimited,apoelstra/secp256k1,evoskuil/secp256k1,cddjr/BitcoinUnlimited,sipa/secp256k1-zkp,marcusdiaz/BitcoinUnlimited,evoskuil/secp256k1,ElementsProject/secp256k1-zkp,vlajos/secp256k1,voisine/secp256k1,libmetrocoin/secp256k1,bitcoin/secp256k1,krzysztofwos/BitcoinUnlimited,pmienk/secp256k1,Justaphf/BitcoinUnlimited,rustyrussell/secp256k1,krzysztofwos/BitcoinUnlimited,martindale/secp256k1-zkp,GSongHashrate/secp256k1,Gustav-Simonsson/secp256k1,ftrader-bitcoinunlimited/hardfork_prototype_1_mvf-bu,marcusdiaz/BitcoinUnlimited,apoelstra/secp256k1,randy-waterhouse/secp256k1,libbitcoin/secp256k1,vlajos/secp256k1,martindale/secp256k1,BitcoinUnlimited/BitcoinUnlimited,martindale/secp256k1,ftrader-bitcoinunlimited/hardfork_prototype_1_mvf-bu,vlajos/secp256k1,rustyrussell/secp256k1,krzysztofwos/BitcoinUnlimited,sipa/secp256k1,shea256/secp256k1-zkp,jonasschnelli/secp256k1,martindale/secp256k1,01BTC10/secp256k1,BitcoinUnlimited/BitcoinUnlimited,voisine/secp256k1,Justaphf/BitcoinUnlimited,martindale/secp256k1-zkp,krzysztofwos/BitcoinUnlimited,afk11/secp256k1,evoskuil/secp256k1,shea256/secp256k1-zkp,peterdettman/secp256k1,bitcoin/secp256k1,Bitcoin-com/BUcash,GSongHashrate/secp256k1,libmetrocoin/secp256k1,afk11/secp256k1,Gustav-Simonsson/secp256k1,rustyrussell/secp256k1,Bitcoin-com/BUcash,Bitcoin-com/BUcash,pmienk/secp256k1,sipa/secp256k1,cddjr/BitcoinUnlimited,Bitcoin-com/BUcash,instagibbs/secp256k1,apoelstra/secp256k1,shea256/secp256k1-zkp,martindale/secp256k1-zkp,01BTC10/secp256k1,ftrader-bitcoinunlimited/hardfork_prototype_1_mvf-bu,marcusdiaz/BitcoinUnlimited,apoelstra/secp256k1,cddjr/BitcoinUnlimited,sipa/secp256k1,ChrisXin/secp256k1,BitcoinUnlimited/BitcoinUnlimited,sipa/secp256k1-zkp,libbitcoin/secp256k1,kleetus/secp256k1,ftrader-bitcoinunlimited/hardfork_prototype_1_mvf-bu,kleetus/secp256k1,chfast/secp256k1,Gustav-Simonsson/secp256k1,randy-waterhouse/secp256k1,krzysztofwos/BitcoinUnlimited,BTCfork/hardfork_prototype_1_mvf-bu,libbitcoin/secp256k1,libmetrocoin/secp256k1,cddjr/BitcoinUnlimited,martindale/secp256k1,marcusdiaz/BitcoinUnlimited,Bitcoin-com/BUcash,sipa/secp256k1-zkp,marcusdiaz/BitcoinUnlimited,afk11/secp256k1,evoskuil/secp256k1,Justaphf/BitcoinUnlimited,vlajos/secp256k1,sipa/secp256k1-zkp,libbitcoin/secp256k1,jonasschnelli/secp256k1,cddjr/BitcoinUnlimited,bitcoin/secp256k1,instagibbs/secp256k1,martindale/secp256k1-zkp,ftrader-bitcoinunlimited/hardfork_prototype_1_mvf-bu,digitalbitbox/secp256k1,BitcoinUnlimited/BitcoinUnlimited,shea256/secp256k1-zkp,Gustav-Simonsson/secp256k1,instagibbs/secp256k1,ChrisXin/secp256k1,pmienk/secp256k1,01BTC10/secp256k1,kleetus/secp256k1,evoskuil/secp256k1,Justaphf/BitcoinUnlimited,BTCfork/hardfork_prototype_1_mvf-bu,Justaphf/BitcoinUnlimited,sipa/secp256k1-zkp,jonasschnelli/secp256k1,peterdettman/secp256k1,GSongHashrate/secp256k1,bitcoin/secp256k1,bitcoin/secp256k1,chfast/secp256k1