cmake: sphinx is a hard dependency for manpages

If the user requests to build manpages by setting
`FREECIV_ENABLE_MANPAGES` to `ON`, `sphinx` is now treated as a hard
dependency.

While here simplify the code and re-add the default option set to `OFF`,
as requested per review.

docs/CMakeLists.txt

@@ -1,6 +1,10 @@
 # CMakeLists for Freeciv21 Docs
 
-find_package(Sphinx QUIET)
+if(FREECIV_ENABLE_MANPAGES)
+  find_package(Sphinx REQUIRED)
+else()
+  find_package(Sphinx QUIET)
+endif()
 
 if(SPHINX_FOUND)
   message(STATUS "Sphinx Found, configuring.")
@@ -14,18 +18,19 @@ if(SPHINX_FOUND)
                 WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}
                 COMMENT "Generating documentation with Sphinx")
 
-  # We only create man pages on Unix
+  # We only create man pages on Unix or if the user explicitly requested it.
   if((UNIX AND NOT APPLE) OR FREECIV_ENABLE_MANPAGES)
     option(FREECIV_ENABLE_MANPAGES "Enable manpages" ON)
 
-    if(FREECIV_ENABLE_MANPAGES)
-      add_custom_target(man-pages ALL
-                  COMMAND
-                  ${SPHINX_EXECUTABLE} -b man ${SPHINX_SOURCE} ${SPHINX_MAN}
-                  WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}
-                  COMMENT "Generating manual pages with Sphinx")
-    endif()
+    add_custom_target(man-pages ALL
+          COMMAND
+          ${SPHINX_EXECUTABLE} -b man ${SPHINX_SOURCE} ${SPHINX_MAN}
+          WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}
+          COMMENT "Generating manual pages with Sphinx")
   endif()
 else()
   message(STATUS "Sphinx NOT Found.")
 endif()
+
+# Default to OFF, if FREECIV_ENABLE_MANPAGES is not set yet.
+option(FREECIV_ENABLE_MANPAGES "Enable manpages" OFF)