[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`.

tools/macos.py

@@ -1,5 +1,6 @@
 import os
 import sys
+import common_compiler_flags
 
 
 def has_osxcross():
@@ -70,3 +71,5 @@ def generate(env):
     )
 
     env.Append(CPPDEFINES=["MACOS_ENABLED", "UNIX_ENABLED"])
+
+    common_compiler_flags.generate(env)