While building RMStore for an app I was working on, I ran into a problem where I was getting these very obtuse errors from Xcode:

Undefined symbols for architecture x86_64:
    "_llvm_gcov_init", referenced from:
    ___llvm_gcov_init in libRMStore.a(RMStore.o)
    ___llvm_gcov_init in libRMStore.a(RMStoreAppReceiptVerifier.o)
    ___llvm_gcov_init in libRMStore.a(RMStoreKeychainPersistence.o)
    ___llvm_gcov_init in libRMStore.a(RMAppReceipt.o)
"_llvm_gcda_end_file", referenced from:
    ___llvm_gcov_writeout in libRMStore.a(RMStore.o)
    ___llvm_gcov_writeout in libRMStore.a(RMStoreAppReceiptVerifier.o)
    ___llvm_gcov_writeout in libRMStore.a(RMStoreKeychainPersistence.o)
    ___llvm_gcov_writeout in libRMStore.a(RMAppReceipt.o)
"_llvm_gcda_emit_function", referenced from:
    ___llvm_gcov_writeout in libRMStore.a(RMStore.o)
    ___llvm_gcov_writeout in libRMStore.a(RMStoreAppReceiptVerifier.o)
    ___llvm_gcov_writeout in libRMStore.a(RMStoreKeychainPersistence.o)
    ___llvm_gcov_writeout in libRMStore.a(RMAppReceipt.o)
"_llvm_gcda_emit_arcs", referenced from:
    ___llvm_gcov_writeout in libRMStore.a(RMStore.o)
    ___llvm_gcov_writeout in libRMStore.a(RMStoreAppReceiptVerifier.o)
    ___llvm_gcov_writeout in libRMStore.a(RMStoreKeychainPersistence.o)
    ___llvm_gcov_writeout in libRMStore.a(RMAppReceipt.o)
"_llvm_gcda_start_file", referenced from:
    ___llvm_gcov_writeout in libRMStore.a(RMStore.o)
    ___llvm_gcov_writeout in libRMStore.a(RMStoreAppReceiptVerifier.o)
    ___llvm_gcov_writeout in libRMStore.a(RMStoreKeychainPersistence.o)
    ___llvm_gcov_writeout in libRMStore.a(RMAppReceipt.o)
"_llvm_gcda_summary_info", referenced from:
    ___llvm_gcov_writeout in libRMStore.a(RMStore.o)
    ___llvm_gcov_writeout in libRMStore.a(RMStoreAppReceiptVerifier.o)
    ___llvm_gcov_writeout in libRMStore.a(RMStoreKeychainPersistence.o)
    ___llvm_gcov_writeout in libRMStore.a(RMAppReceipt.o)
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

This was really strange, since I had successfully built RMStore using a bitcode-enabled OpenSSL before. I immediately took to diffing the build settings for the two different apps and zeroed on the build setting Instrument Program Flow. It had been enabled for debug builds in the bad app, but disabled on the working app. Toggling that to No instantly fixed it.

Some perfunctory Google searches don't yield much helpful information on what this particular build setting is, except that it is related to unit testing and to turn it off.