Failure to link against libmemcached

Issue

• libmemcached fails to link libpthread on RHEL 7

# cat test.c 
#include <libmemcached/memcached.h>

int main()
{
    memcached_st ms;
    memcached_create(&ms);
    return 0;
}

# gcc -Wall -o test test.c -lmemcached
/usr/lib/gcc/x86_64-redhat-linux/4.8.2/../../../../lib64/libmemcached.so: undefined reference to `pthread_once'
collect2: error: ld returned 1 exit status

# ldd /usr/lib64/libmemcached.so
    linux-vdso.so.1 =>  (0x00007fff985fe000)
    libdl.so.2 => /lib64/libdl.so.2 (0x00007fce89122000)
    libsasl2.so.3 => /lib64/libsasl2.so.3 (0x00007fce88f05000)
    libstdc++.so.6 => /lib64/libstdc++.so.6 (0x00007fce88bfd000)
    libm.so.6 => /lib64/libm.so.6 (0x00007fce888fb000)
    libc.so.6 => /lib64/libc.so.6 (0x00007fce8853a000)
    libgcc_s.so.1 => /lib64/libgcc_s.so.1 (0x00007fce88323000)
    /lib64/ld-linux-x86-64.so.2 (0x00007fce8956c000)
    libresolv.so.2 => /lib64/libresolv.so.2 (0x00007fce88109000)
    libcrypt.so.1 => /lib64/libcrypt.so.1 (0x00007fce87ed2000)
    libfreebl3.so => /lib64/libfreebl3.so (0x00007fce87c52000)