fastjet: add an optimization and warning flag when building (#49555)

Co-authored-by: jmcarcell <jmcarcell@users.noreply.github.com>

var/spack/repos/builtin/packages/fastjet/package.py

@@ -148,5 +148,5 @@ def configure_args(self):
 
     def flag_handler(self, name, flags):
         if name == "cxxflags":
-            flags.append(f"-std=c++{self.spec.variants['cxxstd'].value}")
+            flags.append(f"-O2 -Wall -std=c++{self.spec.variants['cxxstd'].value}")
         return (None, flags, None)