Add copy_inc_files function to fix issue of lost pyconfig.h.

When sys.exec_prefix and sys.prefix differ the pyconfig.h file
is in $(exec_prefix)/include/python$(version) and that file is
missing in the virtualenv because env/include/python$(version)
is a symlink to $(real_prefix)/include/python$(version).
If --always-copy is used then the two include directories merge.
I added copy_inc_files to symlink (or copy) the individual contents
of prefix include and exec_prefix include into env/include.