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/libmctp-log.h b/libmctp-log.h
index 1024913..001374b 100644
--- a/libmctp-log.h
+++ b/libmctp-log.h
@@ -4,44 +4,18 @@
 #define _LIBMCTP_LOG_H
 
 /* libmctp-internal logging */
-#ifndef pr_fmt
-#define pr_fmt
-#endif
 
-#if defined(MCTP_LOG_STDERR)
+/* these should match the syslog-standard LOG_* definitions, for
+ * easier use with syslog */
+#define MCTP_LOG_ERR		3
+#define MCTP_LOG_WARNING	4
+#define MCTP_LOG_NOTICE		5
+#define MCTP_LOG_INFO		6
+#define MCTP_LOG_DEBUG		7
 
-#include <stdio.h>
-
-#define MCTP_LOG_ERR		0
-#define MCTP_LOG_WARNING	0
-#define MCTP_LOG_NOTICE		0
-#define MCTP_LOG_INFO		0
-#define MCTP_LOG_DEBUG		0
-
-#define mctp_prlog(x, fmt, ...) fprintf(stderr, fmt "\n", ##__VA_ARGS__)
-
-#elif defined(MCTP_LOG_SYSLOG)
-
-#include <syslog.h>
-
-#define MCTP_LOG_ERR		LOG_ERR
-#define MCTP_LOG_WARNING	LOG_WARNING
-#define MCTP_LOG_NOTICE		LOG_NOTICE
-#define MCTP_LOG_INFO		LOG_INFO
-#define MCTP_LOG_DEBUG		LOG_DEBUG
-
-#define mctp_prlog(x, fmt, ...) syslog(x, fmt, ##__VA_ARGS__)
-
-#elif defined(MCTP_LOG_CUSTOM)
-
-extern void mctp_prlog(int level, const char *fmt, ...)
+void mctp_prlog(int level, const char *fmt, ...)
 	__attribute__((format(printf, 2, 3)));
 
-
-#else
-#error No log implementation found
-#endif
-
 #define mctp_prerr(fmt, ...)  mctp_prlog(MCTP_LOG_ERR, fmt, ##__VA_ARGS__)
 #define mctp_prwarn(fmt, ...) mctp_prlog(MCTP_LOG_WARNING, fmt, ##__VA_ARGS__)
 #define mctp_prinfo(fmt, ...) mctp_prlog(MCTP_LOG_INFO, fmt, ##__VA_ARGS__)