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/llvm-lld/package.py

@@ -38,7 +38,10 @@ class LlvmLld(Package):
     depends_on('cmake', type='build')
 
     def install(self, spec, prefix):
-        env['CXXFLAGS'] = self.compiler.cxx11_flag
+        if 'CXXFLAGS' in env and env['CXXFLAGS']:
+            env['CXXFLAGS'] += ' ' + self.compiler.cxx11_flag
+        else:
+            env['CXXFLAGS'] = self.compiler.cxx11_flag
 
         with working_dir('spack-build', create=True):
             cmake('..',