SCons: Add DEV_ENABLED defines for target=debug builds

This will allow adding developer checks which will be fully compiled out in
user builds, unlike `DEBUG_ENABLED` which is included in debug tempates and
the editor builds.

This define is not used yet, but we'll soon add code that uses it, and change
some existing `DEBUG_ENABLED` checks to be performed only in dev builds.

Related to godotengine/godot-proposals#3371.

platform/android/detect.py

@@ -203,6 +203,7 @@ def configure(env):
         env.Append(LINKFLAGS=["-O0"])
         env.Append(CCFLAGS=["-O0", "-g", "-fno-limit-debug-info"])
         env.Append(CPPDEFINES=["_DEBUG", "DEBUG_ENABLED"])
+        env.Append(CPPDEFINES=["DEV_ENABLED"])
         env.Append(CPPFLAGS=["-UNDEBUG"])
 
     # Compiler configuration