cppflags point to ncurses headers (#12691)

While debugging this package on our Cray machine, tmux was throwing an
error involving the ncurses headers. Tmux would look for headers in
system locations rather than where spack installed ncurses. Tested only
on a Cray.

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

@@ -30,5 +30,10 @@ class Tmux(AutotoolsPackage):
     depends_on('libevent')
     depends_on('ncurses')
 
+    def flag_handler(self, name, flags):
+        if name == 'cppflags':
+            flags.append(self.spec['ncurses'].headers.include_flags)
+        return (None, flags, None)
+
     def configure_args(self):
         return ['LIBTINFO_LIBS=-lncurses']