Add configure support for --with-xdr-include option

[jwpeterson]

RPC headers can be installed in places other than /usr/include/tirpc so let the user specify the correct PATH.

Closes #3709.

[mboisson]

Will this also work from an environment variable ? Unfortunately, some packages bundle libMesh in their build script, and there's no way to change the options passed to configure without patching the build scripts.

[jwpeterson]

Will this also work from an environment variable ?

No this patch won't work with an environment variable as-is. I think I can fix that, but FYI if you are building the libmesh used by MOOSE, it should be possible to pass additional configure flags to the script you call to build libmesh (at least it was possible at one time), so in that case you wouldn't need to patch the build scripts.

[mboisson]

ah, indeed there are options to the script (though I did not see it in the documentation, they just say to run the script as is without referring to any option).

[moosebuild]

Job Coverage on 4031892 wanted to post the following:

The following coverage requirement(s) failed:

• Failed to generate coverage rate (required: 55.0%)