af_alg: Add configure option to enable/disable use of Linux crypto API. Suggested by Assaf Gordon <assafgordon@gmail.com>. * m4/af_alg.m4 (gl_AF_ALG): Add AC_ARG_WITH invocation. Define C macro USE_LINUX_CRYPTO_API. * lib/af_alg.h: Test USE_LINUX_CRYPTO_API, not HAVE_LINUX_IF_ALG_H. * lib/af_alg.c: Likewise.