Makefile.am: fix build, use $(CRYPTO_LIBS)

Use $(CRYPTO_LIBS) instead of -lcryptopp to work also with
nss crypto lib.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
Merge pull request #417 from dalgaaf/wip-da-fix-CRYPTO_LIBS

Makefile.am: fix build, use $(CRYPTO_LIBS)

Reviewed-by: Sage Weil <sage@Inktank.com>