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

.github/workflows/ci.yml

@@ -49,7 +49,7 @@ jobs:
           - name: Ubuntu 18.04 wxGTK 3 with clang
             runner: ubuntu-18.04
             compiler: clang
-            configure_flags: --disable-sys-libs --with-libcurl
+            configure_flags: --disable-sys-libs
             use_xvfb: true
           - name: Ubuntu 20.04 wxGTK ANSI
             runner: ubuntu-20.04
@@ -57,7 +57,7 @@ jobs:
             skip_samples: true
           - name: Ubuntu 20.04 wxGTK with ASAN
             runner: ubuntu-20.04
-            configure_flags: --disable-compat30 --disable-sys-libs --with-libcurl --disable-webview
+            configure_flags: --disable-compat30 --disable-sys-libs --disable-webview
             skip_samples: true
             use_asan: true
             use_xvfb: true