boost: use clang toolset when building with Fujitsu compiler. (#11827)

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

@@ -204,7 +204,8 @@ def determine_toolset(self, spec):
                     'armclang++': 'clang',
                     'xlc++': 'xlcpp',
                     'xlc++_r': 'xlcpp',
-                    'pgc++': 'pgi'}
+                    'pgc++': 'pgi',
+                    'FCC': 'clang'}
 
         if spec.satisfies('@1.47:'):
             toolsets['icpc'] += '-linux'