编译jrtplib和jthread时，出现如下错误：
arm-uclibc-linux-g++ -O2 -o example1 example1.o ../src/.libs/libjrtp.a
../src/.libs/libjrtp.a(rtpsession.o): In function `RTPSession::GetLocalSSRC()':
rtpsession.cpp:(.text+0x58): undefined reference to `JMutex::Lock()'
rtpsession.cpp:(.text+0x88): undefined reference to `JMutex::Unlock()'
../src/.libs/libjrtp.a(rtpsession.o): In function `RTPSession::SendPacket(void const*, unsigned int)':
rtpsession.cpp:(.text+0x2b4): undefined reference to `JMutex::Lock()'
rtpsession.cpp:(.text+0x2c4): undefined reference to `JMutex::Unlock()'
rtpsession.cpp:(.text+0x2dc): undefined reference to `JMutex::Lock()'
rtpsession.cpp:(.text+0x2fc): undefined reference to `JMutex::Unlock()'
rtpsession.cpp:(.text+0x314): undefined reference to `JMutex::Lock()'
rtpsession.cpp:(.text+0x334): undefined reference to `JMutex::Unlock()'
rtpsession.cpp:(.text+0x348): undefined reference to `JMutex::Unlock()'
../src/.libs/libjrtp.a(rtpsession.o): In function `RTPSession::SendPacket(void const*, unsigned int, unsigned char, bool, unsigned int)':
rtpsession.cpp:(.text+0x444): undefined reference to `JMutex::Lock()'
rtpsession.cpp:(.text+0x454): undefined reference to `JMutex::Unlock()'
rtpsession.cpp:(.text+0x464): undefined reference to `JMutex::Unlock()'
rtpsession.cpp:(.text+0x47c): undefined reference to `JMutex::Lock()'
rtpsession.cpp:(.text+0x49c): undefined reference to `JMutex::Unlock()'
rtpsession.cpp:(.text+0x4b4): undefined reference to `JMutex::Lock()'
rtpsession.cpp:(.text+0x4c8): undefined reference to `JMutex::Unlock()'
../src/.libs/libjrtp.a(rtpsession.o): In function `RTPSession::SendPacketEx(void const*, unsigned int, unsigned short, void const*, unsigned int)':
rtpsession.cpp:(.text+0x5c8): undefined reference to `JMutex::Lock()'
rtpsession.cpp:(.text+0x5d8): undefined reference to `JMutex::Unlock()'
rtpsession.cpp:(.text+0x5e8): undefined reference to `JMutex::Unlock()'
rtpsession.cpp:(.text+0x600): undefined reference to `JMutex::Lock()'
rtpsession.cpp:(.text+0x620): undefined reference to `JMutex::Unlock()'
rtpsession.cpp:(.text+0x638): undefined reference to `JMutex::Lock()'
rtpsession.cpp:(.text+0x64c): undefined reference to `JMutex::Unlock()'
../src/.libs/libjrtp.a(rtpsession.o): In function `RTPSession::SendPacketEx(void const*, unsigned int, unsigned char, bool, unsigned int, unsigned short, void const*, unsigned int)':
rtpsession.cpp:(.text+0x76c): undefined reference to `JMutex::Lock()'
rtpsession.cpp:(.text+0x77c): undefined reference to `JMutex::Unlock()'
rtpsession.cpp:(.text+0x78c): undefined reference to `JMutex::Unlock()'
rtpsession.cpp:(.text+0x7a4): undefined reference to `JMutex::Lock()'
rtpsession.cpp:(.text+0x7c4): undefined reference to `JMutex::Unlock()'
rtpsession.cpp:(.text+0x7dc): undefined reference to `JMutex::Lock()'
rtpsession.cpp:(.text+0x7f0): undefined reference to `JMutex::Unlock()'
../src/.libs/libjrtp.a(rtpsession.o): In function `RTPSession::SendRTCPAPPPacket(unsigned char, unsigned char const*, void const*, unsigned int)':
rtpsession.cpp:(.text+0x9a0): undefined reference to `JMutex::Unlock()'
rtpsession.cpp:(.text+0xa64): undefined reference to `JMutex::Unlock()'
rtpsession.cpp:(.text+0xac8): undefined reference to `JMutex::Unlock()'
rtpsession.cpp:(.text+0xae0): undefined reference to `JMutex::Lock()'
rtpsession.cpp:(.text+0xb40): undefined reference to `JMutex::Lock()'
rtpsession.cpp:(.text+0xb5c): undefined reference to `JMutex::Unlock()'
rtpsession.cpp:(.text+0xba8): undefined reference to `JMutex::Lock()'
../src/.libs/libjrtp.a(rtpsession.o): In function `RTPSession::SetDefaultPayloadType(unsigned char)':
rtpsession.cpp:(.text+0xc24): undefined reference to `JMutex::Lock()'
rtpsession.cpp:(.text+0xc34): undefined reference to `JMutex::Unlock()'
../src/.libs/libjrtp.a(rtpsession.o): In function `RTPSession::SetDefaultMark(bool)':