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/Makefile.inc b/Makefile.inc
index fd63032..1ee555d 100644
--- a/Makefile.inc
+++ b/Makefile.inc
@@ -1,5 +1,5 @@
 LIBMCTP_DIR ?= libmctp/
-LIBMCTP_OBJS = core.o alloc.o
+LIBMCTP_OBJS = core.o alloc.o log.o
 LIBMCTP_BINDINGS ?= serial astlpc
 
 LIBMCTP_OBJS += $(LIBMCTP_BINDINGS:%=%.o)