CMake: Fix precompiled Boost libraries on Linux

When building with precompiled libraries on Linux, CMake used boost libs
from the system outside the lib dir. This restricts CMake to use only the
libraries from the precompiled libraries.

Reviewed By: sergey

Differential Revision: https://developer.blender.org/D6659

build_files/cmake/platform/platform_unix.cmake

@@ -53,6 +53,10 @@ if(EXISTS ${LIBDIR})
   set(CMAKE_PREFIX_PATH ${LIBDIR}/zlib ${LIB_SUBDIRS})
   set(WITH_STATIC_LIBS ON)
   set(WITH_OPENMP_STATIC ON)
+  set(Boost_NO_BOOST_CMAKE ON)
+  set(BOOST_ROOT ${LIBDIR}/boost)
+  set(BOOST_LIBRARYDIR ${LIBDIR}/boost/lib)
+  set(Boost_NO_SYSTEM_PATHS ON)
 endif()
 
 if(WITH_STATIC_LIBS)