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/pexsi/make.inc

@@ -36,7 +36,7 @@ DSUPERLU_INCLUDE = -I${DSUPERLU_DIR}/include
 INCLUDES = ${PEXSI_INCLUDE} ${DSUPERLU_INCLUDE}
 
 # Libraries
-CPP_LIB = @STDCXX_LIB @MPICXX_LIB
+CPP_LIB = @MPICXX_LIB @STDCXX_LIB
 #GFORTRAN_LIB = /usr/lib/gcc/x86_64-linux-gnu/4.8/libgfortran.a
 LAPACK_LIB = @LAPACK_LIBS
 BLAS_LIB = @BLAS_LIBS

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

@@ -59,7 +59,7 @@ def install(self, spec, prefix):
             '@MPICC': self.spec['mpi'].mpicc,
             '@MPICXX': self.spec['mpi'].mpicxx,
             '@MPIFC': self.spec['mpi'].mpifc,
-            '@MPICXX_LIB': ' '.join(self.spec['mpi'].mpicxx_shared_libs),
+            '@MPICXX_LIB': self.spec['mpi:cxx'].libs.joined(),
             '@RANLIB': 'ranlib',
             '@PEXSI_STAGE': self.stage.source_path,
             '@SUPERLU_PREFIX': self.spec['superlu-dist'].prefix,
@@ -67,8 +67,9 @@ def install(self, spec, prefix):
             '@PARMETIS_PREFIX': self.spec['parmetis'].prefix,
             '@LAPACK_PREFIX': self.spec['lapack'].prefix,
             '@BLAS_PREFIX': self.spec['blas'].prefix,
-            '@LAPACK_LIBS': self.spec['lapack'].lapack_libs.joined(),
-            '@BLAS_LIBS': self.spec['lapack'].blas_libs.joined(),
+            '@LAPACK_LIBS': self.spec['lapack'].libs.joined(),
+            '@BLAS_LIBS': self.spec['blas'].libs.joined(),
+            # FIXME : what to do with compiler provided libraries ?
             '@STDCXX_LIB': ' '.join(self.compiler.stdcxx_libs)
         }