Fix building on macOS

[skosukhin]

Currently, the configuration fails on macOS with the --disable-libxml2 option:

configure: error: Error: OSX requires libxml2 => --disable-dap4.

The configuration fails even if we add an extra option --disable-dap.

Also, one of the two lines here is redundant: the configuration should either fail or override the user's choice and proceed. Although I personally prefer the former, the latter seems to be a common pattern in the configure script. Therefore, this MR switches from AC_MSG_ERROR to AC_MSG_WARN and modifies the misleading message (there is no option --disable-dap4). It also doesn't make sense to emit the message and set enable_dap4 to no if the variable is already set to no.

I am not sure, whether the problem fixed with 6d44ec3 is still relevant because I have just managed to build the library with the bundled XML implementation.

[DennisHeimbigner]

Thanks for catching this. Ideally, we would fail the build if the user explicitly set some option that
we needed to disable, --enable-dap, in this case, I think. If the option is not set explicitly, then
we try to disable and give a warning. Unfortunately, under current build systems, it can be difficult to tell
if a an explicit option was given. If we give a warning, then it often gets overlooked by the user.
In this case, I think giving a warning is the correct action, as you note.
Also, good to hear that tinyxml2 built under OSX. That would allow us to clean up the configure.ac file a bit.