Set `build_doc`s variables also in the parent scope

src/BuildDoc.cmake

@@ -6,10 +6,16 @@ find_package (
 	OPTIONAL_COMPONENTS mscgen dia
 )
 
+set(DOXYGEN_FOUND ${DOXYGEN_FOUND} PARENT_SCOPE)
+
 set(IHTA_CMAKE_COMMON_DOCSTYLE_LANDED ${CMAKE_CURRENT_LIST_DIR}/resources/landed.css)
 set(IHTA_CMAKE_COMMON_DOCSTYLE_RWTH ${CMAKE_CURRENT_LIST_DIR}/resources/rwth.css)
 set(IHTA_CMAKE_COMMON_DOC_LOGO ${CMAKE_CURRENT_LIST_DIR}/resources/ihta.svg)
 
+set(IHTA_CMAKE_COMMON_DOCSTYLE_LANDED ${IHTA_CMAKE_COMMON_DOCSTYLE_LANDED} PARENT_SCOPE)
+set(IHTA_CMAKE_COMMON_DOCSTYLE_RWTH ${IHTA_CMAKE_COMMON_DOCSTYLE_RWTH} PARENT_SCOPE)
+set(IHTA_CMAKE_COMMON_DOC_LOGO ${IHTA_CMAKE_COMMON_DOC_LOGO} PARENT_SCOPE)
+
 #[=======================================================================[.rst:
 ..command:: build_doc