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/libint/package.py

@@ -85,6 +85,7 @@ def setup_environment(self, build_env, run_env):
     def configure_args(self):
 
         config_args = ['--enable-shared']
+
         optflags = self.optflags
 
         # Optimization flag names have changed in libint 2