Rework conditional feature usage

Currently, the infrastructure that we have to enable a flexible
compilation environment has a few issues:

 - the allocator configuration is performed at run-time, while the log
   configuration is performed at compile-time

 - for a standard compile (ie, standard userspace, using
   autoconf+automake to build), we need a few pre-defined configuration
   options.

This change adds a bit of runtime selection to the logging
infrastructure, to match the allocator setup. We also unify the
compile-time defines into config.h, using MCTP_-prefixed macro names,
allowing integration into other build systems.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
diff --git a/configure.ac b/configure.ac
index 334b0a5..a33d1b8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -15,6 +15,14 @@
 # libtool init
 LT_INIT
 
+# Set defaults for standard library compiles. We may want to
+# AC_ARG_WITH these in the future.
+AC_DEFINE([MCTP_HAVE_SYSLOG], [1], [Define to enable syslog])
+AC_DEFINE([MCTP_HAVE_FILEIO], [1], [Define to enable filesystem functions])
+AC_DEFINE([MCTP_DEFAULT_ALLOC], [1],
+    [Define to populate allocation functions to defaults (malloc/free)])
+
+# Enable all bindings. AC_ARG_ENABLE in future.
 AM_CONDITIONAL([LIBMCTP_BINDING_serial], [true])
 AM_CONDITIONAL([LIBMCTP_BINDING_astlpc], [true])