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/CMakeLists.txt b/CMakeLists.txt
index 83dfafa..6c82849 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -36,9 +36,11 @@
 if((CMAKE_BUILD_TYPE STREQUAL debug) OR (CMAKE_BUILD_TYPE STREQUAL Package))
     option(ENABLE_BUILD_TESTS "Build tests" ON)
     option(ENABLE_VALGRIND_TESTS "Build tests with valgrind" ON)
+    set(INTERNAL_DOCS YES)
 else()
     option(ENABLE_BUILD_TESTS "Build tests" OFF)
     option(ENABLE_VALGRIND_TESTS "Build tests with valgrind" OFF)
+    set(INTERNAL_DOCS NO)
 endif()
 option(ENABLE_COVERAGE "Build code coverage report from tests" OFF)