Add overridable boost_python library name

This would be needed for updating boost to 1.67 or later - the python
bindings libraries changed their names to contain python version they
are built against. Also this naming change is backported to our boost
packages on linux.

CMakeLists.txt

@@ -165,10 +165,15 @@ endif()
 if (PYBINDINGS)
   if (PYTHON_VERSION VERSION_GREATER 3.0)
     set(_Boost_PYTHON3_HEADERS "boost/python.hpp")
-    find_package(Boost 1.64 REQUIRED COMPONENTS python3)
+    if (NOT DEFINED BOOST_PYTHON_LIBNAME)
+      set(BOOST_PYTHON_LIBNAME python3)
+    endif()
   else()
-    find_package(Boost 1.64 REQUIRED COMPONENTS python)
+    if (NOT DEFINED BOOST_PYTHON_LIBNAME)
+      set(BOOST_PYTHON_LIBNAME python)
+    endif()
   endif()
+  find_package(Boost 1.64 REQUIRED COMPONENTS ${BOOST_PYTHON_LIBNAME})
   find_package(PythonLibs ${PYTHON_VERSION} REQUIRED)
   include_directories(${PYTHON_INCLUDE_DIRS})
 endif()