chore: support building libstrophe as a subproject

Allow building the latest libstrophe from upstream Git automatically when
-Dforce_fallback_for=libstrophe is passed.

Default to find and link against system-provided libstrophe stays the same.

Signed-off-by: Paul Fertser <fercerpav@gmail.com>

subprojects/libstrophe.wrap

@@ -0,0 +1,4 @@
+[wrap-git]
+url = https://github.com/strophe/libstrophe.git
+revision = HEAD
+patch_directory = libstrophe