Automake sanitisation

    Do not alter environment CFLAGS, CPPFLAGS etc.
    
    These variables should never be overriden by our makefiles, only the
    AM_CFLAGS or libcf_agent_la_CFLAGS variants are to be set.
    
    Also fixed wrong libutils dependencies that were discovered after this
    change: libutils currently must link to OpenSSL and PCRE.
    
    This clean-up occurred after an attempt to only link cf-upgrade to
    libutils, but noticing that it pulls out everything as dependency.

``ENTERPRISE_CFLAGS`` is a leftover artifact from the pre-enterprise-plugin days. I think it can be removed entirely. But that can be a separate PR.