Replace --enable-webrequest-xxx configure options with --with-xxx

Using --with is more appropriate for libcurl and WinHTTP as they
introduce dependencies on external libraries and is done for
NSURLSession too for consistency.

And there doesn't seem to be any real reason to make these options names
longer by including "webrequest" in their names, they're clear and
unambiguous enough even without it.

docs/gtk/install.md

@@ -148,6 +148,9 @@ minimize external dependencies.
     --without-liblzma       Disable LZMA compression support.
                             Don't use liblzma.
 
+    --without-libcurl       Don't use libcurl even if it's available.
+                            Disables wxWebRequest.
+
     --without-opengl        Disable OpenGL integration with wxGLCanvas.
                             Don't use OpenGL or EGL libraries.