configure: make --disable-docs imply --disable-manual

Because when the docs is not built, the necesary curl.txt file is not
present so then the manual cannot get built.

Reported-by: Harry Sintonen
Closes #13191

configure.ac

@@ -999,48 +999,51 @@ AS_HELP_STRING([--disable-mqtt],[Disable MQTT support]),
 )
 
 dnl **********************************************************************
-dnl Check whether to build documentation
+dnl Check for built-in manual
 dnl **********************************************************************
 
-AC_MSG_CHECKING([whether to build documentation])
-AC_ARG_ENABLE(docs,
-AS_HELP_STRING([--enable-docs],[Enable documentation])
-AS_HELP_STRING([--disable-docs],[Disable documentation]),
+AC_MSG_CHECKING([whether to provide built-in manual])
+AC_ARG_ENABLE(manual,
+AS_HELP_STRING([--enable-manual],[Enable built-in manual])
+AS_HELP_STRING([--disable-manual],[Disable built-in manual]),
 [ case "$enableval" in
   no)
        AC_MSG_RESULT(no)
-       BUILD_DOCS=0
-       curl_docs_msg="no"
        ;;
   *)   AC_MSG_RESULT(yes)
-       BUILD_DOCS=1
+       USE_MANUAL="1"
        ;;
   esac ],
        AC_MSG_RESULT(yes)
-       BUILD_DOCS=1
+       USE_MANUAL="1"
 )
+dnl The actual use of the USE_MANUAL variable is done much later in this
+dnl script to allow other actions to disable it as well.
 
 dnl **********************************************************************
-dnl Check for built-in manual
+dnl Check whether to build documentation
 dnl **********************************************************************
 
-AC_MSG_CHECKING([whether to provide built-in manual])
-AC_ARG_ENABLE(manual,
-AS_HELP_STRING([--enable-manual],[Enable built-in manual])
-AS_HELP_STRING([--disable-manual],[Disable built-in manual]),
+AC_MSG_CHECKING([whether to build documentation])
+AC_ARG_ENABLE(docs,
+AS_HELP_STRING([--enable-docs],[Enable documentation])
+AS_HELP_STRING([--disable-docs],[Disable documentation]),
 [ case "$enableval" in
   no)
        AC_MSG_RESULT(no)
+       BUILD_DOCS=0
+       dnl disable manual too because it needs built documentation
+       USE_MANUAL=0
+       curl_docs_msg="no"
        ;;
   *)   AC_MSG_RESULT(yes)
-       USE_MANUAL="1"
+       BUILD_DOCS=1
        ;;
   esac ],
        AC_MSG_RESULT(yes)
-       USE_MANUAL="1"
+       BUILD_DOCS=1
 )
-dnl The actual use of the USE_MANUAL variable is done much later in this
-dnl script to allow other actions to disable it as well.
+
 
 dnl ************************************************************
 dnl disable C code generation support