Check signature nonces for validity

The documentation implies that this check is happening, so make it so.
Without this check, passing an invalid nonce will trigger an internal assertion.
Fix x64 gmp init on platforms that define UL as 32 bits.
Merge pull request #9

ac274bf Fix x64 gmp init on platforms that define UL as 32 bits. (evoskuil)
Merge pull request #8

ba8fc0e Check signature nonces for validity (William Swanson)
Add MacPorts default include/lib paths for OSX
add pkg-config support
packaging: remove the --with-pkgconfigdir option

It was not necessary and would only confuse packagers.
packaging: fixup pkg-config

Set the private libs as determined by configure, and add the missing lib link.
packaging: add pkg-config file to gitignore
secp256k1_fe_sqrt checks for success

- secp256k1_fe_sqrt now checks that the value it calculated is actually a square root.
- Add return values to secp256k1_fe_sqrt and secp256k1_ge_set_xo.
- Callers of secp256k1_ge_set_xo can use return value instead of explicit validity checks
- Add random value tests for secp256k1_fe_sqrt
Use endomorphism in precomputations
Documented autotools build process in readme
Support 64bit_asm field on OSX
Merge pull request #23

17eec03 Support 64bit_asm field on OSX (Peter Dettman)
Merge pull request #22

7fc1fb4 Documented autotools build process in readme (John Driscoll)
Merge pull request #20

d7fd4d0 Use endomorphism in precomputations (Peter Dettman)
Merge pull request #19

09ca4f3 secp256k1_fe_sqrt checks for success (Peter Dettman)
Merge pull request #17

a86f241 Add MacPorts default include/lib paths for OSX (Peter Dettman)
A few improvements to the sqrt tests
Rewrite 5x52 normalize method to be faster