Readline package: Add ncurses library directory to link line. (#3802)

Resolves a issue with finding the ncurses library on MacOS.

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

@@ -45,4 +45,8 @@ class Readline(AutotoolsPackage):
     patch('readline-6.3-upstream_fixes-1.patch', when='@6.3')
 
     def build(self, spec, prefix):
-        make('SHLIB_LIBS=-lncursesw')
+        options = [
+            'SHLIB_LIBS=-L{0} -lncursesw'.format(spec['ncurses'].prefix.lib)
+        ]
+
+        make(*options)