concretizer: only minimize builds when --reuse is enabled.

Make the first minimization conditional on whether `--reuse` is enabled in the solve.
If `--reuse` is not enabled, there will be nothing in the set to minimize and the
objective function (for this criterion) will be 0 for every answer set.

lib/spack/spack/solver/asp.py

@@ -1570,6 +1570,8 @@ def setup(self, driver, specs, tests=False, reuse=False):
 
         if reuse:
             self.gen.h1("Installed packages")
+            self.gen.fact(fn.optimize_for_reuse())
+            self.gen.newline()
             self.define_installed_packages(possible)
 
         self.gen.h1('General Constraints')

lib/spack/spack/solver/concretize.lp

@@ -794,7 +794,8 @@ build(Package) :- not hash(Package, _), node(Package).
 % Try hard to reuse installed packages (i.e., minimize the number built)
 opt_criterion(16, "number of packages to build (vs. reuse)").
 #minimize { 0@16: #true }.
-#minimize { 1@16,Package : build(Package) }.
+#minimize { 1@16,Package : build(Package), optimize_for_reuse() }.
+#defined optimize_for_reuse/0.
 
 % Minimize the number of deprecated versions being used
 opt_criterion(15, "deprecated versions used").