rrdtool: only unset PERLHOSTLIB in do_configure

The do_compile requires it, otherwise it can't find the native module
correctly when perl-native is mirrored from another build.

Fixed:
| Compilation failed in require.
| BEGIN failed--compilation aborted.
| Can't locate loadable object for module File::Glob in @INC

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
(cherry picked from commit daf619ec4cffab45f0137d02a770071643d10432)
Signed-off-by: Armin Kuster <akuster808@gmail.com>