postpone LIBPATH setting as other tools might modify environment

Change-Id: I39f48db544f6a2edb1dbf9a681a92bd8e2d6b507

SConsider/site_tools/OutputDirectoryHelper.py

@@ -135,6 +135,7 @@ def verifyBaseoutDirWritable(baseoutdir):
 
 def prePackageCollection(env, **kw):
     sconstruct_dir = kw.get('sconstruct_dir', Dir('#'))
+    env.AppendUnique(LIBPATH=[getLibraryInstallDir(env)])
     env.AppendUnique(EXCLUDE_DIRS_REL=[env['BUILDDIR']])
     if env.getBaseOutDir() == sconstruct_dir:
         env.AppendUnique(EXCLUDE_DIRS_TOPLEVEL=[
@@ -201,8 +202,6 @@ def generate(env):
     env.AddMethod(getLogInstallDir, 'getLogInstallDir')
     env.AddMethod(makeInstallablePathFromDir, 'makeInstallablePathFromDir')
 
-    env.AppendUnique(LIBPATH=[getLibraryInstallDir(env)])
-
     Callback().register('PrePackageCollection', prePackageCollection)