Besides the proposed answer to convert someURL in text to the formula
HYPERLINK("someURL"), the manual way I found to make Calc recognize a URL in text is to edit the cell in the formula bar, add a space on the end of the URL, delete it, and press [Enter] or click the ✓ to complete your change (that didn’t change anything). As soon as I add a space the URL turns blue-on-gray in the formula bar indicating Calc “spotted” the URL.
This trick doesn’t work if you’re editing in the default in-place [F2] Cell Edit Mode; Calc : How to edit formulae in formula bar directly explains how you can define a keyboard shortcut for the Calc Application function “Input Line”, or redefine the [F2] key from “Cell Edit Mode” to this.
Whether Calc will recognize URLs in text that you paste into it seems hit-or-miss in Linux. I think it depends on whether the incoming text is on the clipboard in a rich text format or HTML format.