libxpm: cope with system gettext if configured. (#10881)

* If gettext is installed in the system it won't be in the spec tree.

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

@@ -27,5 +27,9 @@ class Libxpm(AutotoolsPackage):
     depends_on('util-macros', type='build')
 
     def setup_environment(self, spack_env, run_env):
-        spack_env.append_flags('LDFLAGS', '-L{0} -lintl'.format(
-            self.spec['gettext'].prefix.lib))
+        # If libxpm is installed as an external package, gettext won't
+        # be available in the spec. See
+        # https://github.com/spack/spack/issues/9149 for details.
+        if 'gettext' in self.spec:
+            spack_env.append_flags('LDFLAGS', '-L{0} -lintl'.format(
+                self.spec['gettext'].prefix.lib))