Some of the compilation commands have CFLAGS/CXXFLAGS missing, with the result that linkage fails with a missing external symbol '__sync_fetch_and_add_4'. In particular, the -march option is missing, which causes GCC to "downgrade" to using 80386 instructions, which do not include "atomic" instructions; this means that atomic operations have to be performed under a bus lock. Note that this is not as big a problem on AMD64 processors, as even the earliest of that architecture have atomic instructions as standard. emerge --info and the ebuild log will follow shortly, as attachments.
Created attachment 248018 [details] emerge --info details
Created attachment 248020 [details] ebuild log (zipped)
Oh, I see. third_party/protobuf2 seems to not respect user's CFLAGS. Could you file an upstream bug (http://new.crbug.com) and post the link here? For reference the final error message is: out/Release/obj.host/protoc/third_party/protobuf2/src/src/google/protobuf/compiler/code_generator.o: In function `google::protobuf::compiler::ParseGeneratorParameter(std::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, std::vector<std::pair<std::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::allocator<std::pair<std::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::basic_string<char, std::char_traits<char>, std::allocator<char> > > > >*)': code_generator.cc:(.text._ZN6google8protobuf8compiler23ParseGeneratorParameterERKSsPSt6vectorISt4pairISsSsESaIS6_EE+0x25f): undefined reference to `__sync_fetch_and_add_4' out/Release/obj.host/protoc/third_party/protobuf2/src/src/google/protobuf/compiler/code_generator.o:code_generator.cc:(.text._ZN6google8protobuf8compiler23ParseGeneratorParameterERKSsPSt6vectorISt4pairISsSsESaIS6_EE+0x29c): more undefined references to `__sync_fetch_and_add_4' follow collect2: ld returned 1 exit status
The upstream report is: http://code.google.com/p/chromium/issues/detail?id=56210