Fix an issue when `-U in build_flags does not remove macro previously defined via -D` flag // Resolve #2508

HISTORY.rst

@@ -33,6 +33,7 @@ PlatformIO 4.0
 * **Miscellaneous**
 
   - Deprecated ``--only-check`` PlatformIO Core CLI option for "update" sub-commands, please use ``--dry-run`` instead
+  - Fixed an issue when ``-U`` in ``build_flags`` does not remove macro previously defined via ``-D`` flag (`issue #2508 <https://github.com/platformio/platformio-core/issues/2508>`_)
 
 PlatformIO 3.0
 --------------

platformio/builder/tools/platformio.py

@@ -198,6 +198,8 @@ def ProcessFlags(env, flags):  # pylint: disable=too-many-branches
     if undefines:
         for undef in undefines:
             env['CCFLAGS'].remove(undef)
+            if undef[2:] in env['CPPDEFINES']:
+                env['CPPDEFINES'].remove(undef[2:])
         env.Append(_CPPDEFFLAGS=" %s" % " ".join(undefines))