add and use a version.c / version.h pair

version.c is now object code which is recompiled each time
ssrf-version.h changes, while the interface file version.h
remains that same at all times and files which include it
will not need to be recompiled.

Signed-off-by: Lubomir I. Ivanov <neolit123@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

version.c

@@ -0,0 +1,16 @@
+#include "ssrf-version.h"
+
+const char *subsurface_version(void)
+{
+	return VERSION_STRING;
+}
+
+const char *subsurface_git_version(void)
+{
+	return GIT_VERSION_STRING;
+}
+
+const char *subsurface_canonical_version(void)
+{
+	return CANONICAL_VERSION_STRING;
+}