cmake: Remove Boost::system from component list

This has multiple reasons:

- We use CMake's FindBoost
  (https://cmake.org/cmake/help/v3.12/module/FindBoost.html) which can
  resolve dependencies itself. We only use boost::system in tandem with
  Boost::asio, which we already list as a dependency.
- There are combinations of CMake versions and Boost versions, where
  specifying this will link against the stub library of Boost::system
  (this is a header-only library since Boost 1.69). Removing it from the
  list avoids any kind of linkage.

Note that UHDBoost.cmake has some compatibility hacks for Boost::system,
which we now also remove.

host/CMakeLists.txt

@@ -320,8 +320,8 @@ set(UHD_BOOST_REQUIRED_COMPONENTS
     date_time
     filesystem
     program_options
-    system
     serialization
+    #system Boost.System is used, but explicitly listing it causes errors on some systems.
     thread
     unit_test_framework
 )
@@ -591,9 +591,9 @@ endif(NOT CMAKE_MODULES_DIR)
 list(APPEND UHD_LINK_LIST_STATIC "dl")
 list(APPEND UHD_LINK_LIST_STATIC "pthread")
 foreach(Boost_Comp ${UHD_BOOST_REQUIRED_COMPONENTS})
-    if(NOT ${Boost_Comp} STREQUAL "unit_test_framework")
+    if(NOT ${Boost_Comp} STREQUAL "unit_test_framework" AND NOT ${Boost_Comp} STREQUAL "system")
         list(APPEND UHD_LINK_LIST_STATIC "boost_${Boost_Comp}")
-    endif(NOT ${Boost_Comp} STREQUAL "unit_test_framework")
+    endif()
 endforeach(Boost_Comp)
 if(ENABLE_USB)
     list(APPEND UHD_LINK_LIST_STATIC "usb-1.0")

host/cmake/Modules/UHDBoost.cmake

@@ -100,14 +100,6 @@ if(MINGW)
     endif()
 endif()
 
-# if 'system' is in the list, make sure it comes last; sometimes this
-# linkage makes a difference
-list(FIND UHD_BOOST_REQUIRED_COMPONENTS "system" SYSTEM_NDX)
-if(NOT ${SYSTEM_NDX} EQUAL -1)
-    list(REMOVE_AT UHD_BOOST_REQUIRED_COMPONENTS ${SYSTEM_NDX})
-    list(APPEND UHD_BOOST_REQUIRED_COMPONENTS "system")
-endif()
-
 # special library directory that's used by some Linux
 if(UNIX AND NOT BOOST_ROOT AND EXISTS "/usr/lib64")
     list(APPEND BOOST_LIBRARYDIR "/usr/lib64") #fedora 64-bit fix

host/lib/CMakeLists.txt

@@ -182,7 +182,6 @@ target_link_libraries(uhd
     Boost::date_time
     Boost::filesystem
     Boost::serialization
-    Boost::system
     Boost::thread
     ${libuhd_libs}
     PRIVATE uhd_rc

host/python/CMakeLists.txt

@@ -154,11 +154,11 @@ target_include_directories(pyuhd PUBLIC
 )
 
 if(WIN32)
-    target_link_libraries(pyuhd ${Boost_LIBRARIES} ${PYTHON_LIBRARIES} uhd)
+    target_link_libraries(pyuhd ${PYTHON_LIBRARIES} uhd)
 else()
     # for extension module, proper to NOT link against python library and instead
     # add dynamic lookup link option on OSX
-    target_link_libraries(pyuhd ${Boost_LIBRARIES} uhd)
+    target_link_libraries(pyuhd uhd)
     if(APPLE)
         target_link_options(pyuhd PRIVATE "LINKER:-undefined,dynamic_lookup")
     endif(APPLE)