new package: Percept (#6636)

This also adds the 'exception' boost library to the set of default
libraries built by boost.

var/spack/repos/builtin/packages/boost/package.py

@@ -91,6 +91,7 @@ class Boost(Package):
     default_install_libs = set(['atomic',
                                 'chrono',
                                 'date_time',
+                                'exception',
                                 'filesystem',
                                 'graph',
                                 'iostreams',
@@ -339,6 +340,8 @@ def install(self, spec, prefix):
             withLibs.remove('chrono')
         if not spec.satisfies('@1.43.0:'):
             withLibs.remove('random')
+        if not spec.satisfies('@1.39.0:'):
+            withLibs.remove('exception')
         if '+graph' in spec and '+mpi' in spec:
             withLibs.append('graph_parallel')