Fix pkg-config Libs.private entry

Index: CMakeLists.txt
--- CMakeLists.txt.orig
+++ CMakeLists.txt
@@ -121,11 +121,7 @@ endif()
 
 include(CheckCXXSymbolExists)
 check_cxx_symbol_exists(_LIBCPP_VERSION cstdlib HAVE_LIBCPP)
-if(HAVE_LIBCPP)
-  set(LIBS_PRIVATE "-lc++")
-else()
-  set(LIBS_PRIVATE "-lstdc++")
-endif()
+set(LIBS_PRIVATE "-lpthread -lm")
 
 
 option(BUILD_SHARED_LIBS "Build shared library" ON)
