petsc: fix suitesparse configure options. (#11188)

spec['dep'].libs.ld_flags can return system paths in some cases.
This is a targeted fix for the petsc package when retrieving
suite-sparse dependency libraries.

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

@@ -303,7 +303,7 @@ def install(self, spec, prefix):
                 'camd,amd,suitesparseconfig'
             options.extend([
                 '--with-suitesparse-include=%s' % spec[ss_spec].prefix.include,
-                '--with-suitesparse-lib=%s' % spec[ss_spec].libs.ld_flags,
+                '--with-suitesparse-lib=%s' % spec[ss_spec].libs.joined(),
                 '--with-suitesparse=1'
             ])
         else: