[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/web.py

@@ -1,4 +1,5 @@
 import os
+import common_compiler_flags
 from SCons.Util import WhereIs
 
 
@@ -42,3 +43,5 @@ def generate(env):
     env.Append(LINKFLAGS=["-s", "SIDE_MODULE=1"])
 
     env.Append(CPPDEFINES=["WEB_ENABLED", "UNIX_ENABLED"])
+
+    common_compiler_flags.generate(env)