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

subprojects/packagefiles/libstrophe/meson.build

@@ -0,0 +1,19 @@
+project('libstrophe', 'c',
+  meson_version : '>=0.60.3',
+  version : run_command('git', 'describe',  '--tags', check : true).stdout().strip(),
+)
+
+mod = import('unstable-external_project')
+
+run_command('bootstrap.sh', check : true)
+
+p = mod.add_project('configure',
+  configure_options : ['--prefix=@PREFIX@',
+                       '--libdir=@PREFIX@/@LIBDIR@',
+                       '--includedir=@PREFIX@/@INCLUDEDIR@',
+                      ],
+  verbose : true
+)
+
+libstrophe_dep = p.dependency('strophe')
+meson.override_dependency('libstrophe', libstrophe_dep)