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.
CFLAGS, CPPFLAGS etc should override auto-detected variables.

CFLAGS, CPPFLAGS, LDFLAGS are the ones that user sets on the command
line by doing:

./configure CFLAGS=-O3

Thus they should stay the last ones, overriding all auto-detected values
and configure-specified values. This patch attempts to make this happen.

Nevertheless we still retain and manually append ENV_CFLAGS at the end
of CFLAGS.  Even though CFLAGS should contain the ./configure command
line CFLAGS as last, some macro seem to mess the order up and insert
its own optimisation flags as well. So we append ENV_CFLAGS at the end
manually, causing a bit of flag duplication.  Thanks @kacfengine for
bringing this up.
Separately include proper CPPFLAGS for every binary

Now that CORE_*FLAGS are not used anymore, I tried to explicitly include
only the needed CPPFLAGS for each binary. Unfortunately, because
cf3.defs.h is inluded almost everywhere, and it includes headers from
libutils, which needs OpenSSL includes, I had to carry the
OPENSSL_CPPFLAGS almost everywhere, and in fewer cases PCRE_CPPFLAGS as
well.

I didn't manage to completely remove those dependencies (OpenSSL, PCRE),
but at least now they are not included from within platform.h, but only
from the respective headers that need them.

To sum up, the dependency mess is less but still exists, but at least it
is visible in each makefile and we can untangle it little by little. And
every new dependency added by mistake will trigger a failure, where
previously it succeeded because CORE_CPPFLAGS included everything plus
the kitchen sink.
Remove hackishly stated dependencies, they are properly detected now.
Include stddef.h always

Fixing the broken Solaris build.
Removed unused struct, and the openssl/rsa.h dependency from cf3.defs.h.
Fix detection of getline() which has been a GNU extension before POSIX.1-2008.
Fix ./configure -C on platforms without proper printf.
Remove poinless type qualifier.

It makes zero difference to the compiler since the returned struct may
freely be assigned to a const or not struct. It also saves a bunch of
warnings when -Wextra is employed.