Don't require python exe and lib versions to match while crosscompiling

[lanza]

Summary

While cross compiling, the python executable is used to run a handful
of scripts while the libraries are linked and headers are included.
Theoretically it's possible for the versions to match completely, but
requiring the build to match 2.7.10 to 2.7.15 is unnecessary.

lldb/trunk/cmake/modules/LLDBConfig.cmake

if (NOT LLDB_RELOCATABLE_PYTHON)
       file(TO_CMAKE_PATH "${PYTHON_HOME}" LLDB_PYTHON_HOME)
       add_definitions( -DLLDB_PYTHON_HOME="${LLDB_PYTHON_HOME}" )
     endif()
   else()
     find_package(PythonInterp REQUIRED)
     find_package(PythonLibs REQUIRED)
   endif()
 
-  if (NOT PYTHON_VERSION_STRING VERSION_EQUAL PYTHONLIBS_VERSION_STRING)
+  if (NOT PYTHON_VERSION_STRING VERSION_EQUAL PYTHONLIBS_VERSION_STRING AND
+      NOT CMAKE_CROSSCOMPILING)
     message(FATAL_ERROR "Found incompatible Python interpreter (${PYTHON_VERSION_STRING}) and Python libraries (${PYTHONLIBS_VERSION_STRING})")
   endif()
 
   if (PYTHON_INCLUDE_DIR)
     include_directories(${PYTHON_INCLUDE_DIR})
   endif()
 endif()