Allow packages to control handling of compiler flags (#4421)

* Initial work on flag trapping using functions called <flag>_handler and default_flag_handler

* Update packages so they do not obliterate flags

* Added append to EnvironmentModifications class

* changed EnvironmentModifications to have append_flags method

* changed flag_val to be a tuple

* Increased test coverage

* added documentation of flag handling

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

@@ -71,8 +71,15 @@ def install(self, spec, prefix):
             if which('xiar'):
                 env['AR'] = 'xiar'
 
-        env['CFLAGS']  = optflags
-        env['FCFLAGS'] = optflags
+        if 'CFLAGS' in env and env['CFLAGS']:
+            env['CFLAGS'] += ' ' + optflags
+        else:
+            env['CFLAGS'] = optflags
+
+        if 'FCFLAGS' in env and env['FCFLAGS']:
+            env['FCFLAGS'] += ' ' + optflags
+        else:
+            env['FCFLAGS'] = optflags
 
         configure('--prefix={0}'.format(prefix),
                   '--enable-shared')