meson: Do not add compiler flags if 'plain' buildtype is used

That is how the 'plain' buildtype is meant in meson.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/464

meson.build

@@ -167,6 +167,8 @@ cdata.set('HAVE_FDWALK',
           cc.has_function('fdwalk')
 )
 
+buildtype = get_option('buildtype')
+if buildtype != 'plain'
   all_warnings = [
     '-fno-strict-aliasing',
     '-Wpointer-arith',
@@ -204,10 +206,9 @@ all_warnings = [
     '-Werror=empty-body',
     '-Werror=write-strings',
   ]
-
   supported_warnings = cc.get_supported_arguments(all_warnings)
-
   add_project_arguments(supported_warnings, language: 'c')
+endif
 
 if get_option('debug')
   debug_c_args = [