build fix

[victoryeo]

fix build on Ubuntu 20.04

[un-fuf-a-doo]

Sorry, but these patches actually break the cmake process.

-- Looking for pthread.h - not found
-- Could NOT find Threads (missing: Threads_FOUND)
CMake Error at /usr/share/cmake-3.20/Modules/FindPackageHandleStandardArgs.cmake:230 (message):
  Could NOT find Boost (missing: thread) (found suitable version "1.65.1",
  minimum required is "1.55")
Call Stack (most recent call first):
  /usr/share/cmake-3.20/Modules/FindPackageHandleStandardArgs.cmake:594 (_FPHSA_FAILURE_MESSAGE)
  /usr/share/cmake-3.20/Modules/FindBoost.cmake:2344 (find_package_handle_standard_args)
  CMakeLists.txt:112 (find_package)


-- Configuring incomplete, errors occurred!

Even copying pthread.h into the source tree has no effect.