Nextgen objc link fix

Our projects (mix of C/C++/ASM/ObjC/Objc++) fails to link on OSX with current NextGen. I have provided a simple sample program that exhibits this problem (tests/objc), before the changes to macosx.jam the build output is as follows:

@ C.clang.C++ <macosx32!release:objc>main.o 
@ C.clang.Link <macosx32!release:objc>objc 

	"/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang++" -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.9.sdk -gdwarf-2 -arch i386 -framework AppKit -o ./macosx32-release/objc/objc.release ./macosx32-release/objc/main.o  

Undefined symbols for architecture i386:
  "_OBJC_CLASS_$_NSAlert", referenced from:
      objc-class-ref in main.o
ld: symbol(s) not found for architecture i386
clang: error: linker command failed with exit code 1 (use -v to see invocation)