cmdline-docs: fix make install with configure --disable-docs
make -C docs/cmdline-opts install depends on all-am, which in turn
depends on $(MANS), unconditionally defined to be $(man_MANS).
As with CLEANFILES, only add curl.1 to man_MANS when BUILD_DOCS is true
so we don't try to build curl.1 unnecessarily.
Closes #13198