build: make tests optional

Add a meson option -Dtests and --without-tests automake option
to disable the compilation of all available testcases.
This is useful for compiling projects with Flatpak such as
GNOME Control Center which disables all possible integrations since they
only need the DBus part of ModemManager.

Contributes to https://gitlab.gnome.org/GNOME/gnome-control-center/-/merge_requests/1392

meson.build

@@ -88,6 +88,9 @@ cc_args = cc.get_supported_arguments([
   '-Wno-missing-field-initializers',
 ])
 
+# tests are enabled by default
+enable_tests = get_option('tests')
+
 # strict flags to use in debug builds
 if get_option('buildtype').contains('debug')
   cc_args += cc.get_supported_arguments([
@@ -266,7 +269,9 @@ if dist_version != ''
   config_h.set('MM_DIST_VERSION', dist_version)
 endif
 
-util_dep = cc.find_library('util')
+if enable_tests
+  util_dep = cc.find_library('util')
+endif
 
 # introspection support
 enable_gir = get_option('introspection')
@@ -376,14 +381,19 @@ subdir('introspection')
 subdir('include')
 
 subdir('libqcdm/src')
-subdir('libqcdm/tests')
+if enable_tests
+  subdir('libqcdm/tests')
+endif
 
 subdir('libmm-glib')
 subdir('src')
 subdir('plugins')
 subdir('cli')
-subdir('test')
-subdir('tools/tests')
+
+if enable_tests
+  subdir('test')
+  subdir('tools/tests')
+endif
 
 subdir('examples/sms-c')