CMake: Mark Doxygen as required and use DOXYGEN_EXECUTABLE

Seems like I missed that when adding doxygen support.

Gbp-Dch: ignore

doc/CMakeLists.txt

@@ -39,14 +39,14 @@ add_docbook(libapt-pkg-doc HTML TEXT ALL
     DEPENDS ${ENTITIES}
 )
 
-find_package(Doxygen)
+find_package(Doxygen REQUIRED)
 
 file(GLOB_RECURSE apt_pkg "../apt-pkg/*.h" "../apt-pkg/*.cc")
 configure_file(Doxyfile.in ${CMAKE_CURRENT_BINARY_DIR}/Doxyfile)
 
 add_custom_command(
     OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/doxygen.stamp
-    COMMAND doxygen ${CMAKE_CURRENT_BINARY_DIR}/Doxyfile
+    COMMAND ${DOXYGEN_EXECUTABLE} ${CMAKE_CURRENT_BINARY_DIR}/Doxyfile
     COMMAND ${CMAKE_COMMAND} -E touch ${CMAKE_CURRENT_BINARY_DIR}/doxygen.stamp
     DEPENDS ${apt_pkg} ${CMAKE_CURRENT_BINARY_DIR}/Doxyfile
 )