git + luafilesystem: minor fixes (#5635)

* git: setup_dependent_environment won't fail for external packages

* luafilesystem: relax the request on git (it seemed overly restrictive)

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

@@ -161,8 +161,12 @@ class Git(AutotoolsPackage):
     depends_on('m4',       type='build')
 
     def setup_environment(self, spack_env, run_env):
-        spack_env.append_flags('LDFLAGS', '-L{0} -lintl'.format(
-            self.spec['gettext'].prefix.lib))
+        # This is done to avoid failures when git is an external package.
+        # In that case the node in the DAG gets truncated and git DOES NOT
+        # have a gettext dependency.
+        if 'gettext' in self.spec:
+            spack_env.append_flags('LDFLAGS', '-L{0} -lintl'.format(
+                self.spec['gettext'].prefix.lib))
 
     def configure_args(self):
         spec = self.spec

var/spack/repos/builtin/packages/lua-luafilesystem/package.py

@@ -40,7 +40,12 @@ class LuaLuafilesystem(Package):
 
     version('1_6_3', 'bed11874cfded8b4beed7dd054127b24')
 
-    depends_on('git@2.9.3:', type='build')
+    # The version constraint here comes from this post:
+    #
+    # https://www.perforce.com/blog/git-beyond-basics-using-shallow-clones
+    #
+    # where it is claimed that full shallow clone support was added @1.9
+    depends_on('git@1.9.0:', type='build')
     extends('lua')
 
     def install(self, spec, prefix):