Fix interaction between magnitudes and negation

Magnitude m means values are allowed to be up to 2 * 0xFFF...FFF * m,
while the argument passed to secp256k1_fe_negate didn't take the 2 into
account. Fix this.
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)
Get rid of {num,scalar,ecdsa_sig}_{init,free}
Merge pull request #89

501d58f Get rid of {num,scalar,ecdsa_sig}_{init,free} (Pieter Wuille)
Merge pull request #85

7a8e385 Fix interaction between magnitudes and negation (Pieter Wuille)