more succinctly

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

@@ -57,7 +57,7 @@ def install(self, spec, prefix):
         if '+openmp' in spec:
             # Note: Apple's Clang does not support OpenMP.
             if spec.satisfies('%clang'):
-              ver = '%s' % self.compiler.version
+              ver = str(self.compiler.version)
               if ver.endswith('-apple'):
                 raise InstallError("Apple's clang does not support OpenMP")
             options.append('--enable-openmp')