build: Add check for required Crypto++ library

Change-Id: I3e4db0e3b3d2c00f0a66dda0c69c4325b836b75e
build: Conditional enabling of osx-private-key-storage compilation

Makefile.am now contains a condition to enable/disable compilation of
osx-private-key-storage, instead of relying on define in the key storage
implementation.

Change-Id: I4a2079d99cd515c744cbe8f90a34a39e5c8197fa
build: Fixing gcc compilation