New interface for passing build information among specs (#1875)

- Added a new interface for Specs to pass build information
  - Calls forwarded from Spec to Package are now explicit
  - Added descriptor within Spec to manage forwarding
  - Added state in Spec to maintain query information
  - Modified a few packages (the one involved in spack install pexsi) to showcase changes

- This uses an object wrapper to `spec` to implement the `libs` sub-calls.
  - wrapper is returned from `__getitem__` only if spec is concrete
  - allows packagers to access build information easily

var/spack/repos/builtin/packages/superlu-dist/package.py

@@ -53,7 +53,7 @@ class SuperluDist(Package):
     depends_on('metis@5:')
 
     def install(self, spec, prefix):
-        lapack_blas = spec['lapack'].lapack_libs + spec['blas'].blas_libs
+        lapack_blas = spec['lapack'].libs + spec['blas'].libs
         makefile_inc = []
         makefile_inc.extend([
             'PLAT         = _mac_x',
@@ -61,17 +61,17 @@ def install(self, spec, prefix):
             'DSUPERLULIB  = $(DSuperLUroot)/lib/libsuperlu_dist.a',
             'BLASDEF      = -DUSE_VENDOR_BLAS',
             'BLASLIB      = %s' % lapack_blas.ld_flags,
-            'METISLIB     = -L%s -lmetis' % spec['metis'].prefix.lib,
-            'PARMETISLIB  = -L%s -lparmetis' % spec['parmetis'].prefix.lib,
+            'METISLIB     = %s' % spec['metis'].libs.ld_flags,
+            'PARMETISLIB  = %s' % spec['parmetis'].libs.ld_flags,
             'FLIBS        =',
             'LIBS         = $(DSUPERLULIB) $(BLASLIB) $(PARMETISLIB) $(METISLIB)',  # noqa
             'ARCH         = ar',
             'ARCHFLAGS    = cr',
             'RANLIB       = true',
             'CC           = {0}'.format(self.spec['mpi'].mpicc),
-            'CFLAGS       = -fPIC -std=c99 -O2 -I%s -I%s %s' % (
-                spec['parmetis'].prefix.include,
-                spec['metis'].prefix.include,
+            'CFLAGS       = -fPIC -std=c99 -O2 %s %s %s' % (
+                spec['parmetis'].cppflags,
+                spec['metis'].cppflags,
                 '-D_LONGINT' if '+int64' in spec else ''),
             'NOOPTS       = -fPIC -std=c99',
             'FORTRAN      = {0}'.format(self.spec['mpi'].mpif77),