casper: fix compilation errors on newer compilers (#30142)

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

@@ -22,6 +22,11 @@ class Casper(MakefilePackage):
 
     conflicts('%gcc@7.1.0')
 
+    def flag_handler(self, name, flags):
+        if name.lower() == 'cxxflags':
+            flags.append(self.compiler.cxx98_flag)
+        return (flags, None, None)
+
     def install(self, spec, prefix):
         install_tree('.', prefix)