[SCons] Split targets.py, apply flags from tools

Split `targets` tool logic, moving all the compiler-specific flags to a
new `common_compiler_flags.py` file, and everything else (CPPDEFINES,
optimize option logic, dev build logic, etc) to the `godotcpp` tool.

The default tools now apply the common compiler flags by importing the
file and explicitly calling `configure`.

(cherry picked from commit 16df4bff30)

tools/android.py

@@ -1,6 +1,7 @@
 import os
 import sys
 import my_spawn
+import common_compiler_flags
 from SCons.Script import ARGUMENTS
 
 
@@ -118,3 +119,5 @@ def generate(env):
     env.Append(LINKFLAGS=["--target=" + arch_info["target"] + env["android_api_level"], "-march=" + arch_info["march"]])
 
     env.Append(CPPDEFINES=["ANDROID_ENABLED", "UNIX_ENABLED"])
+
+    common_compiler_flags.generate(env)