Compilation error when building with -DCMAKE_BUILD_TYPE=Release

[lattice0]

cmake -DCMAKE_BUILD_TYPE=Release .

Then I get

/home/lz/orwell/myRtspClient/third_party/JRTPLIB/src/rtprandomurandom.cpp: In member function ‘virtual uint8_t jrtplib::RTPRandomURandom::GetRandom8()’:
/home/lz/orwell/myRtspClient/third_party/JRTPLIB/src/rtprandomurandom.cpp:71:7: error: ignoring return value of ‘size_t fread(void*, size_t, size_t, FILE*)’, declared with attribute warn_unused_result [-Werror=unused-result]
  fread(&value, sizeof(uint8_t), 1, device);
  ~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/lz/orwell/myRtspClient/third_party/JRTPLIB/src/rtprandomurandom.cpp: In member function ‘virtual uint16_t jrtplib::RTPRandomURandom::GetRandom16()’:
/home/lz/orwell/myRtspClient/third_party/JRTPLIB/src/rtprandomurandom.cpp:83:7: error: ignoring return value of ‘size_t fread(void*, size_t, size_t, FILE*)’, declared with attribute warn_unused_result [-Werror=unused-result]
  fread(&value, sizeof(uint16_t), 1, device);
  ~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/lz/orwell/myRtspClient/third_party/JRTPLIB/src/rtprandomurandom.cpp: In member function ‘virtual uint32_t jrtplib::RTPRandomURandom::GetRandom32()’:
/home/lz/orwell/myRtspClient/third_party/JRTPLIB/src/rtprandomurandom.cpp:95:7: error: ignoring return value of ‘size_t fread(void*, size_t, size_t, FILE*)’, declared with attribute warn_unused_result [-Werror=unused-result]
  fread(&value, sizeof(uint32_t), 1, device);
  ~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/lz/orwell/myRtspClient/third_party/JRTPLIB/src/rtprandomurandom.cpp: In member function ‘virtual double jrtplib::RTPRandomURandom::GetRandomDouble()’:
/home/lz/orwell/myRtspClient/third_party/JRTPLIB/src/rtprandomurandom.cpp:107:7: error: ignoring return value of ‘size_t fread(void*, size_t, size_t, FILE*)’, declared with attribute warn_unused_result [-Werror=unused-result]
  fread(&value, sizeof(uint64_t), 1, device);
  ~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

I'm confused, I've never compiled for Release, only Debug. I just accidentally turned Release on and got these errors. I understand what they mean but now I'm asking myself how it's possible that these errors have not been fixed? Haven't nobody ever compiled for Release?

[j0r1]

On my system it compiles without any problem, it probably depends on compiler version or installed header files.