message: Fix compilation with Xcode 12.5+
  Mark manager.cpp as containing Objective-C++.
  Fixes
diff: B28F1CC316F629D30018AF7E /* findframe.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; path = findframe.h; sourceTree = "<group>"; };
  B28F1CC416F629D30018AF7E /* gexecute.cpp */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.cpp; path = gexecute.cpp; sourceTree = "<group>"; };
  B28F1CC516F629D30018AF7E /* gexecute.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; path = gexecute.h; sourceTree = "<group>"; };
  - B28F1CCA16F629D30018AF7E /* manager.cpp */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.cpp; path = manager.cpp; sourceTree = "<group>"; };
  + B28F1CCA16F629D30018AF7E /* manager.cpp */ = {isa = PBXFileReference; explicitFileType = sourcecode.cpp.objcpp; fileEncoding = 4; path = manager.cpp; sourceTree = "<group>"; };
  B28F1CCB16F629D30018AF7E /* manager.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; path = manager.h; sourceTree = "<group>"; };
  B28F1CCC16F629D30018AF7E /* macos_helpers.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; path = macos_helpers.h; sourceTree = "<group>"; };
  B28F1CCD16F629D30018AF7E /* macos_helpers.mm */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.objcpp; path = macos_helpers.mm; sourceTree = "<group>"; };
  