build: new option to build plugins within the daemon binary

Instead of creating libmm-plugin* and libmm-shared* libraries that are
dlopen()-ed on runtime, allow incorporating all plugins into the
daemon binary itself.

This makes the startup of the daemon much faster and also avoids
issues with builds that require linker namespace isolation.

Fixes https://gitlab.freedesktop.org/mobile-broadband/ModemManager/-/issues/674

data/tests/meson.build

@@ -1,9 +1,14 @@
 # SPDX-License-Identifier: GPL-2.0-or-later
 # Copyright (C) 2022 Aleksander Morgado <aleksander@aleksander.es>
 
+test_plugin_dir = ''
+if not enable_builtin_plugins
+  test_plugin_dir = '--test-plugin-dir="' + build_root + '/src/plugins"'
+endif
+
 test_conf = {
   'abs_top_builddir': build_root,
-  'PLUGIN_BUILD_SUBDIR': 'src/plugins/',
+  'test_plugin_dir': test_plugin_dir,
 }
 
 configure_file(

data/tests/org.freedesktop.ModemManager1.service.in

@@ -2,4 +2,4 @@
 
 [D-BUS Service]
 Name=org.freedesktop.ModemManager1
-Exec=@abs_top_builddir@/src/ModemManager --test-session --no-auto-scan --test-enable --test-plugin-dir="@abs_top_builddir@/@PLUGIN_BUILD_SUBDIR@" --debug
+Exec=@abs_top_builddir@/src/ModemManager --test-session --no-auto-scan --test-enable @test_plugin_dir@ --debug