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/linuxbsd/detect.py

@@ -113,6 +113,7 @@ def configure(env):
     elif env["target"] == "debug":
         env.Prepend(CCFLAGS=["-g3"])
         env.Prepend(CPPDEFINES=["DEBUG_ENABLED"])
+        env.Prepend(CPPDEFINES=["DEV_ENABLED"])
         env.Append(LINKFLAGS=["-rdynamic"])
 
     ## Architecture