Make --disable-sys-libs configure option less drastic

Only disable using the system libraries when a built-in version is
available, otherwise still use the system library as this seems more
useful than disabling the use of only some system libraries (liblzma and
libcurl), while still possibly depending on dozens more via GTK.

Update the documentation and remove the now unnecessary --with-libcurl
from the CI script.

Closes https://github.com/wxWidgets/wxWidgets/pull/2208

docs/gtk/install.md

@@ -127,10 +127,11 @@ libraries, wxWidgets also provides built-in versions, that can be
 linked into wx libraries themselves, which can be useful to
 minimize external dependencies.
 
-    --disable-sys-libs      Don't use system libraries at all.
-                            Use built-in ones when possible or
-                            disable support for the corresponding
-                            feature otherwise.
+    --disable-sys-libs      Don't use system libraries when there is
+                            a built-in version included in wxWidgets.
+                            Note that other system libraries can,
+                            and typically will, still be used if
+                            found.
 
     --without-libpng        Disables PNG image format code.
                             Don't use libpng (although GTK