obs/slapos: fix perl @INC path.

I found that include paths of slapos package's perl were wrong (i.e. `/usr/src/packages/BUILD/SLAPOS/build` should be removed).

```
~> /opt/slapos/parts/perl/bin/perl -e 'print @INC,"\n"'
/usr/src/packages/BUILD/slapos/build/opt/slapos/parts/site_perl/lib/site_perl/5.20.2/x86_64-linux-thread-multi/usr/src/packages/BUILD/slapos/build/opt/slapos/parts/site_perl/lib/site_perl/5.20.2/usr/src/packages/BUILD/slapos/build/opt/slapos/parts/perl/lib/5.20.2/x86_64-linux-thread-multi/usr/src/packages/BUILD/slapos/build/opt/slapos/parts/perl/lib/5.20.2.
```

I think that this MR fixes this issue but not so sure. Can you please verify it ?

/cc @alain.takoudjou @rafael 

/reviewed-on nexedi/slapos.package!23

obs/slapos/slapos-node/slapos/buildout.cfg.in

@@ -123,5 +123,16 @@ configure-options =
 environment +=
  BISON_PKGDATADIR=${bison:location}/share/bison/
 
+[perl]
+configure-command =
+  sh Configure -des \
+    -Dprefix=${buildout:rootdir}/parts/${:_buildout_section_name_} \
+    -Dsiteprefix=${buildout:rootdir}/parts/site_${:_buildout_section_name_} \
+    -Dcflags=-I${gdbm:location}/include \
+    -Dldflags="-L${gdbm:location}/lib -Wl,-rpath=${gdbm:location}/lib" \
+    -Ui_db \
+    -Dnoextensions=ODBM_File \
+    -Dusethreads
+
 [versions]
 cliff = 1.4.5