build DOC prepare framework for optional building of internal documentation

Prepare framework to put documentation about internal libyang processes
(intended for libyang developers, not its users) into the generated
doxygen documentation. The internal documentation will be available in
the Devel build, Release build is with user documentation only.
diff --git a/Doxyfile.in b/Doxyfile.in
index 5bb0916..2288465 100644
--- a/Doxyfile.in
+++ b/Doxyfile.in
@@ -506,7 +506,7 @@
 # will be excluded. Set it to YES to include the internal documentation.
 # The default value is: NO.
 
-INTERNAL_DOCS          = NO
+INTERNAL_DOCS          = @INTERNAL_DOCS@
 
 # If the CASE_SENSE_NAMES tag is set to NO then doxygen will only generate file
 # names in lower-case letters. If set to YES, upper-case letters are also