build FEATURE new 'format' and 'format-check' make targets

To format source codes (tools are not covered for now) using uncrustify.
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 60e209b..41e017a 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -14,6 +14,7 @@
 include(CheckSymbolExists)
 include(UseCompat)
 include(ABICheck)
+include(SourceFormat)
 
 set(LIBYANG_DESCRIPTION "libyang is YANG data modelling language parser and toolkit written (and providing API) in C.")
 
@@ -300,6 +301,9 @@
 	libyang_abicheck()
 endif()
 
+# source code format
+source_format(src/* compat/*)
+
 # clean cmake cache
 add_custom_target(cclean
         COMMAND make clean