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/zlib/package.py

@@ -57,7 +57,7 @@ def libs(self):
 
     def setup_environment(self, spack_env, run_env):
         if '+pic' in self.spec:
-            spack_env.set('CFLAGS', self.compiler.pic_flag)
+            spack_env.append_flags('CFLAGS', self.compiler.pic_flag)
 
     def install(self, spec, prefix):
         config_args = []