[sledge] Build models without threads support

Summary:
There are many assumptions on the behavior of mutexes, condition
variables, etc. in the implementation of the cxxabi with threads
support. So compile with `_LIBCXXABI_HAS_NO_THREADS` defined to select
the much simpler code paths for the single-threaded case.

Reviewed By: kren1

Differential Revision: D16069454

fbshipit-source-id: 9f975e0e6

sledge/model/Makefile

@@ -20,7 +20,7 @@ CLANG_ARGS += --sysroot=/usr
 endif
 
 cxxabi.bc : cxxabi.cpp
-	$(LLVM)/bin/clang $(CLANG_ARGS) -I$(LLVM)/include/c++/v1 -I$(LIBCXXABI)/include -I$(LIBCXXABI)/src -c -emit-llvm cxxabi.cpp
+	$(LLVM)/bin/clang $(CLANG_ARGS) -D_LIBCXXABI_HAS_NO_THREADS -I$(LLVM)/include/c++/v1 -I$(LIBCXXABI)/include -I$(LIBCXXABI)/src -c -emit-llvm cxxabi.cpp
 
 lib_fuzzer_main.bc : lib_fuzzer_main.c
 	$(LLVM)/bin/clang $(CLANG_ARGS) -c -emit-llvm -o $@ $<