macosx: locate the best newest SDK present on the computer.

With recent versions of XCode, PC/SC headers are hidden inside
the SDK folder inside XCode app, and the global path is not
guaranteed to be present. This allows to build easily from source
without depending on a specific XCode/OSX version.
macosx: locate and target the latest SDK to build against.
macosx: update the minimal building instructions.
Use correct bit when writing PIV certificate object with gzipped certificate

NIST 800-73 clarified the CertInfo flag for gzipped certificate is 0x01
piv-tool was writting 0x80.
OpenSC card-piv.c continues to accept both.
Merge pull request #324 from dengert/piv-gzip-cert

Use correct bit when writing PIV certificate object with gzipped certifi...
Merge pull request #308 from martinpaljak/yosemite

Building on newer OSX versions without xcode command line tools
CardOS 5 driver for OpenSC

This change builds upon existing CardOS 4 code to implement a CardOS
5 driver for OpenSC. CardOS 4 and 5 differ substantially. CardOS 5
supports on-card generation and storage of RSA keys up to 4096 bits
in modulus, and ECC up to 512 bits.

With a lot of feedback and guidance from szikora@.
Support large (> 2048-bit) keys.
Add onepin bits to CardOS 4's profile.

When the onepin profile is activated, the ACL of the DF needs to be
adjusted from SOPIN to PIN.
Add brainpoolP512r1.
Set key_info->params.len for SC_ALGORITHM_EC as well.
Export sc_pkcs15init_get_pin_reference().