Cause missing mod_watchdog to 'unset' the --enable-proxy default, rather than

Cause missing mod_watchdog to 'unset' the --enable-proxy default, rather than
disable the module. This forces the module logic to emit a warning of the
missing dependency; changes the output from

checking whether to enable mod_proxy_hcheck... no

to

configure: WARNING: "mod_watchdog is disabled but required for mod_proxy_hcheck"
checking whether to enable mod_proxy_hcheck... no (disabled)

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1749678 13f79535-47bb-0310-9956-ffa450edef68