Add a fallback PKCS5_PBKDF2_HMAC for systems with OpenSSL < 1.0.0.
Continue using the OS X fallback path where available.

If neither options are available, error out early instead of leaving it
to fail at link-time.

Signed-off-by: Thomas Hurst <tom@hur.st>
Tabify and adopt appropriate brace style.

Signed-off-by: Thomas Hurst <tom@hur.st>
Don't build PBKDF2 fallback if we're not going to use it (OS X).

Signed-off-by: Thomas Hurst <tom@hur.st>
Whitespace.

Signed-off-by: Thomas Hurst <tom@hur.st>
Merge pull request #44 from Freaky/master

Add a fallback PKCS5_PBKDF2_HMAC for systems with OpenSSL < 1.0.0.