Compile AU sources with -ObjC++

DPF's DistrhoPluginAU.cpp uses Objective-C++ syntax (NSString,
[obj method] sends, @Class) despite the .cpp extension, and
DistrhoUIMain.cpp transitively pulls in DistrhoUIAU.mm when built
with -DDISTRHO_PLUGIN_TARGET_AU. Both fail to compile as plain C++
on macOS the moment they hit <Foundation/Foundation.h>.

Add target-specific BUILD_CXX_FLAGS += -ObjC++ for the two _AU.cpp.o
targets. Target-specific augmentation, not modification of the
generic pattern rule, so the _VST2 / _VST3 / _CLAP / _LV2 / _JACK
variants keep being compiled as plain C++. Matches upstream
Makefile.plugins.mk:601,605.

Author: magnetophon

plugin/dpf/Makefile.plugins.mk

@@ -231,6 +231,16 @@ $(BUILD_DIR)/DistrhoUIMain_%.cpp.o: $(DPF_PATH)/distrho/DistrhoUIMain.cpp
 	@echo "Compiling DistrhoUIMain.cpp ($*)"
 	@$(CXX) $< $(BUILD_CXX_FLAGS) -DDISTRHO_PLUGIN_TARGET_$* -c -o $@
 
+# The AU variants of DistrhoPluginMain.cpp and DistrhoUIMain.cpp pull in
+# DistrhoPluginAU.cpp / DistrhoUIAU.mm which contain Objective-C++ code
+# (NSString, [obj method] syntax, @class). Compile them with -ObjC++ so
+# clang treats them as Objective-C++ regardless of the .cpp extension.
+# Target-specific BUILD_CXX_FLAGS additions take effect only for these
+# two specific .o targets; the generic _% pattern rule above sees the
+# augmented flags when it expands for $*=AU.
+$(BUILD_DIR)/DistrhoPluginMain_AU.cpp.o: BUILD_CXX_FLAGS += -ObjC++
+$(BUILD_DIR)/DistrhoUIMain_AU.cpp.o: BUILD_CXX_FLAGS += -ObjC++
+
 $(BUILD_DIR)/DistrhoUI_macOS_%.mm.o: $(DPF_PATH)/distrho/DistrhoUI_macOS.mm
 	-@mkdir -p $(BUILD_DIR)
 	@echo "Compiling DistrhoUI_macOS.mm ($*)"