build: Don't append multiple flags as a string

The gtk_debug_cflags variable is an array, with each argument stored in
a separate string; if we add multiple arguments inside the same string
we break the escaping rules.

Fixes: #3864

meson.build

@@ -66,7 +66,7 @@ if debug
     gtk_debug_cflags += '-DG_ENABLE_CONSISTENCY_CHECKS'
   endif
 elif optimization in ['2', '3', 's']
-  gtk_debug_cflags += '-DG_DISABLE_CAST_CHECKS -DG_DISABLE_ASSERT'
+  gtk_debug_cflags += ['-DG_DISABLE_CAST_CHECKS', '-DG_DISABLE_ASSERT']
 endif
 
 add_project_arguments(gtk_debug_cflags, language: 'c')