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/ferret/package.py

@@ -98,7 +98,12 @@ def install(self, spec, prefix):
         ln('-sf',
            libz_prefix + '/lib',
            libz_prefix + '/lib64')
-        os.environ['LDFLAGS'] = '-lquadmath'
+
+        if 'LDFLAGS' in env and env['LDFLAGS']:
+            env['LDFLAGS'] += ' ' + '-lquadmath'
+        else:
+            env['LDFLAGS'] = '-lquadmath'
+
         with working_dir('FERRET', create=False):
             os.environ['LD_X11'] = '-L/usr/lib/X11 -lX11'
             os.environ['HOSTTYPE'] = 'x86_64-linux'