Tmux should look for tinfo in -ltinfo (#15282)

Just `depend_on('ncurses')`, pkgconfig seems to cause the right thing
to happen if the package is built +termlib or ~termlib (tested both
ways on a CentOS 7 system).

Additional details in: https://github.com/spack/spack/issues/15281

Tested by building all of the releases of tmux on a CentOS 7 box.

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

@@ -29,13 +29,7 @@ class Tmux(AutotoolsPackage):
     version('2.1', sha256='31564e7bf4bcef2defb3cb34b9e596bd43a3937cad9e5438701a81a5a9af6176')
     version('1.9a', sha256='c5e3b22b901cf109b20dab54a4a651f0471abd1f79f6039d79b250d21c2733f5')
 
+    # used by configure to e.g. find libtinfo
+    depends_on('pkgconfig', type='build')
     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']