build: make docs by default when configured

Problem: In the old build system, docs were built by default with
`make`, but that is no longer the case with the CMake based build.

Add the ALL keyword to the docs target so docs are built by default
when configured.

Fixes #1091

doc/CMakeLists.txt

@@ -47,6 +47,6 @@ if(SPHINX AND ENABLE_DOCS)
         ${Python_EXECUTABLE} -m sphinx -b html ${CMAKE_CURRENT_SOURCE_DIR} ./html
         )
     add_custom_target(html DEPENDS ${man5_html_outputs})
-    add_custom_target(docs
+    add_custom_target(docs ALL
         DEPENDS html manpages)
 endif()