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
Merge pull request #70

e2d66a2 Fix build for 64bit field under OSX (Peter Dettman)