link to pthreads as well

No idea why and how it was working till today. It got uncovered when I
was playing with the CMakeFile for redis versioning thing.

CMakeLists.txt

@@ -343,6 +343,7 @@ add_executable(parser_tests
   unittests/Parser/ParserTests.cpp
 )
 
+find_package(Threads)
 set(LLVM_LDFLAGS "${LLVM_LDFLAGS} ${ALIVE_LDFLAGS}")
 foreach(target souper internal-solver-test lexer-test parser-test souper-check count-insts
                souperExtractor souperInfer souperInst souperKVStore souperParser
@@ -377,9 +378,9 @@ target_link_libraries(parser-test souperParser)
 target_link_libraries(souper-check souperTool souperExtractor souperKVStore souperSMTLIB2 souperParser ${HIREDIS_LIBRARY} ${ALIVE_LIBRARY} ${Z3_LIBRARY})
 target_link_libraries(clang-souper souperClangTool souperExtractor souperKVStore souperParser souperSMTLIB2 souperTool kleeExpr ${CLANG_LIBS} ${LLVM_LIBS} ${LLVM_LDFLAGS} ${HIREDIS_LIBRARY} ${ALIVE_LIBRARY} ${Z3_LIBRARY})
 target_link_libraries(count-insts souperParser)
-target_link_libraries(extractor_tests souperExtractor ${GTEST_LIBS} ${ALIVE_LIBRARY})
-target_link_libraries(inst_tests souperInst ${GTEST_LIBS} ${ALIVE_LIBRARY})
-target_link_libraries(parser_tests souperParser ${GTEST_LIBS} ${ALIVE_LIBRARY})
+target_link_libraries(extractor_tests souperExtractor ${GTEST_LIBS} ${ALIVE_LIBRARY} ${CMAKE_THREAD_LIBS_INIT})
+target_link_libraries(inst_tests souperInst ${GTEST_LIBS} ${ALIVE_LIBRARY} ${CMAKE_THREAD_LIBS_INIT})
+target_link_libraries(parser_tests souperParser ${GTEST_LIBS} ${ALIVE_LIBRARY} ${CMAKE_THREAD_LIBS_INIT})
 
 SET(BUILD_CLANG_TOOL 1 CACHE BOOL "Build the Souper Clang tool")
 if (NOT BUILD_CLANG_TOOL)