filter_system_path: additionally filter system prefixes (#2672)

That's because in set_build_environment_variables()
the funciton filter_system_paths() is actually applied to
package prefixes, and not to prefix/lib or prefix/include.

lib/spack/spack/test/environment.py

@@ -64,11 +64,14 @@ def test_filter_system_paths(self):
         filtered = filter_system_paths([
             '/usr/local/Cellar/gcc/5.3.0/lib',
             '/usr/local/lib',
+            '/usr/local',
             '/usr/local/include',
             '/usr/local/lib64',
             '/usr/local/opt/some-package/lib',
             '/usr/opt/lib',
             '/lib',
+            '/',
+            '/usr',
             '/lib64',
             '/include',
             '/opt/some-package/include',

lib/spack/spack/util/environment.py

@@ -24,9 +24,10 @@
 ##############################################################################
 import os
 
-system_paths = ['/', '/usr/', '/usr/local']
+system_paths = ['/', '/usr', '/usr/local']
 suffixes = ['lib', 'lib64', 'include']
-system_dirs = [os.path.join(p, s) for s in suffixes for p in system_paths]
+system_dirs = [os.path.join(p, s) for s in suffixes for p in system_paths] + \
+    system_paths
 system_bins = [os.path.join(p, 'bin') for p in system_paths]