FEATURE: library logger

Mechanism for logging messages and setting library specific errno.
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 168b1d7..9b1107a 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -20,6 +20,8 @@
 set(CMAKE_C_FLAGS_DEBUG   "-g -O0")
 
 set(src
+	src/common.c
+	src/log.c
 	src/xml/xml_parser.c)
 
 add_library(yang SHARED ${src})