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/libxpm/package.py

@@ -46,5 +46,5 @@ class Libxpm(AutotoolsPackage):
     depends_on('util-macros', type='build')
 
     def setup_environment(self, spack_env, run_env):
-        spack_env.set('LDFLAGS', '-L{0} -lintl'.format(
+        spack_env.append_flags('LDFLAGS', '-L{0} -lintl'.format(
             self.spec['gettext'].prefix.lib))