Problem: build API incompatible with 4.1

Solution: keep the new --without-docs option, but also keep the old
--without-documentation with an added deprecation warning.
We can then remove it in the next major release, to leave enough time
for users and maintainers to change it without disruptions.

acinclude.m4

@@ -95,8 +95,13 @@ AC_DEFUN([LIBZMQ_CHECK_DOC_BUILD], [{
         AS_HELP_STRING([--without-docs],
             [Don't build and install man pages [default=build]]),
         [with_docs=$withval])
+    AC_ARG_WITH([documentation], [AS_HELP_STRING([--without-documentation],
+        [Don't build and install man pages [default=build] DEPRECATED: use --without-docs])])
 
-    if test "x$with_docs" = "xno"; then
+    if test "x$with_documentation" = "xno"; then
+        AC_MSG_WARN([--without-documentation is DEPRECATED and will be removed in the next release, use --without-docs])
+    fi
+    if test "x$with_docs" = "xno" || test "x$with_documentation" = "xno"; then
         libzmq_build_doc="no"
         libzmq_install_man="no"
     else