Fix: Get rid of absolute paths in Doxygen documentation

When the Doxygen documentation output was moved to the OpENer_BINARY_DIR
by accident the absolute paths of some files were recorded in the
documentation. To avoid this run Doxygen in the OpENer_SOURCE_DIR which
is then implicitly stripped from absolute input paths.

Signed-off-by: Stefan Mätje <stefan.maetje@esd.eu>

source/CMakeLists.txt

@@ -112,7 +112,7 @@ if (DOXYGEN_FOUND)
     # note the option ALL which will build the docs always with the default target
     add_custom_target( doc # ALL
         COMMAND ${DOXYGEN_EXECUTABLE} ${DOXYGEN_OUT}
-        WORKING_DIRECTORY ${OpENer_BINARY_DIR}
+        WORKING_DIRECTORY ${OpENer_SOURCE_DIR}
         COMMENT "Generating API documentation with Doxygen"
         VERBATIM )
     message("-- Doxygen setup done")