Stolen from NixOS packages... later they removed the patch but looks 
like this is what should fix bug https://bugs.gentoo.org/918956

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 73d5c3c84..e4b0dbfd1 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -104,13 +104,6 @@ option(SOUFFLE_CUSTOM_GETOPTLONG "Enable/Disable custom getopt_long implementati
 cmake_dependent_option(SOUFFLE_USE_LIBCPP "Link to libc++ instead of libstdc++" ON
     "CMAKE_CXX_COMPILER_ID STREQUAL Clang" OFF)
 
-# Using Clang? Likely want to use `lld` too.
-if (CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
-    set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -fuse-ld=lld")
-    set(CMAKE_MODULE_LINKER_FLAGS "${CMAKE_MODULE_LINKER_FLAGS} -fuse-ld=lld")
-    set(CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS} -fuse-ld=lld")
-endif()
-
 # Add aditional modules to CMake
 set(CMAKE_MODULE_PATH "${PROJECT_SOURCE_DIR}/cmake" ${CMAKE_MODULE_PATH})
 
@@ -247,7 +240,11 @@ endif()
 # pthreads
 # --------------------------------------------------
 set(THREADS_PREFER_PTHREAD_FLAG ON)
-find_package(Threads REQUIRED)
+set(CMAKE_THREAD_LIBS_INIT "-lpthread")
+set(CMAKE_HAVE_THREADS_LIBRARY 1)
+set(CMAKE_USE_WIN32_THREADS_INIT 0)
+set(CMAKE_USE_PTHREADS_INIT 1)
+set(THREADS_PREFER_PTHREAD_FLAG ON)
 
 # --------------------------------------------------
 # OpenMP