seek /opt/pkg (pkgsrc OS X) for openssl and lz4

Add /opt/pkg (used by pkgsrc on Mac OS X) to possible openssl and
lz4 header path.

    from 2015Q2 onwards the prefix has changed to /opt/pkg
    in order to be compatible with El Capitan's
    "System Integrity Protection" feature.

-- pkgsrc.joynet.com

thanks to @weakish for finding this!

setup.py

@@ -106,7 +106,8 @@ def detect_lz4(prefixes):
 include_dirs = []
 library_dirs = []
 
-possible_openssl_prefixes = ['/usr', '/usr/local', '/usr/local/opt/openssl', '/usr/local/ssl', '/usr/local/openssl', '/usr/local/borg', '/opt/local']
+possible_openssl_prefixes = ['/usr', '/usr/local', '/usr/local/opt/openssl', '/usr/local/ssl', '/usr/local/openssl',
+                             '/usr/local/borg', '/opt/local', '/opt/pkg', ]
 if os.environ.get('BORG_OPENSSL_PREFIX'):
     possible_openssl_prefixes.insert(0, os.environ.get('BORG_OPENSSL_PREFIX'))
 ssl_prefix = detect_openssl(possible_openssl_prefixes)
@@ -116,7 +117,8 @@ include_dirs.append(os.path.join(ssl_prefix, 'include'))
 library_dirs.append(os.path.join(ssl_prefix, 'lib'))
 
 
-possible_lz4_prefixes = ['/usr', '/usr/local', '/usr/local/opt/lz4', '/usr/local/lz4', '/usr/local/borg', '/opt/local']
+possible_lz4_prefixes = ['/usr', '/usr/local', '/usr/local/opt/lz4', '/usr/local/lz4',
+                         '/usr/local/borg', '/opt/local', '/opt/pkg', ]
 if os.environ.get('BORG_LZ4_PREFIX'):
     possible_lz4_prefixes.insert(0, os.environ.get('BORG_LZ4_PREFIX'))
 lz4_prefix = detect_lz4(possible_lz4_prefixes)