Enable depends_on('pcre') also

While I have the patient on the operating table, @adamjstewart asked
me to uncomment and test the depends_on('pcre').

Did it and it Works For Me(tm).

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

@@ -53,10 +53,8 @@ class Git(Package):
     depends_on("expat")
     depends_on("gettext")
     depends_on("zlib")
-
-    # Use system perl for now.
+    depends_on("pcre")
     depends_on("perl")
-    # depends_on("pcre")
 
     def install(self, spec, prefix):
         configure_args = [