added package : pexsi (#1550)

* pexsi : added package
* cp2k : added pexsi (works for gcc+openmpi with and without plumed)

lib/spack/spack/compilers/gcc.py

@@ -83,3 +83,7 @@ def fc_version(cls, fc):
     @classmethod
     def f77_version(cls, f77):
         return cls.fc_version(f77)
+
+    @property
+    def stdcxx_libs(self):
+        return ('-lstdc++', )

lib/spack/spack/compilers/intel.py

@@ -80,3 +80,7 @@ def default_version(cls, comp):
         """
         return get_compiler_version(
             comp, '--version', r'\((?:IFORT|ICC)\) ([^ ]+)')
+
+    @property
+    def stdcxx_libs(self):
+        return ('-cxxlib', )