compile error libsqleet

[hujianglang]

ubuntu 18.04
gcc build sqleet into libsqleet.so (sqleet version : sqleet-v0.31.1-amalgamation.zip)
but error：
./libsqleet.so: undefined reference to dlopen' ./libsqleet.so: undefined reference to pthread_mutexattr_settype'
./libsqleet.so: undefined reference to dlclose' ./libsqleet.so: undefined reference to pthread_mutex_trylock'
./libsqleet.so: undefined reference to dlerror' ./libsqleet.so: undefined reference to dlsym'
./libsqleet.so: undefined reference to pthread_join' ./libsqleet.so: undefined reference to pthread_create'
./libsqleet.so: undefined reference to pthread_mutexattr_init' ./libsqleet.so: undefined reference to pthread_mutexattr_destroy'
i dont why.