The project does not compile cleanly with newer versions of Clang. It appears that Clang has tightened up its checking that C++ templates have C++ linkage:
In file included from /Users/joe/src/plcrashreporter/Source/PLCrashSignalHandler.mm:32:
In file included from /Users/joe/src/plcrashreporter/Source/PLCrashFrameWalker.h:46:
In file included from /Users/joe/src/plcrashreporter/Source/PLCrashAsyncImageList.h:48:
/Users/joe/src/plcrashreporter/Source/PLCrashAsyncLinkedList.hpp:53:1: error: templates must have C++ linkage
template <typename V>
From there, the remaining errors seem to be cascades from that.