perl: identify dependency on gdbm (#3896)

* perl: provide +gdbm variant for use when system dbm is missing or buggy

* perl: remove gdbm variant; always depends on gdbm

* perl: pass gdbm paths as Configure arguments

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

@@ -58,6 +58,8 @@ class Perl(Package):  # Perl doesn't use Autotools, it should subclass Package
 
     extendable = True
 
+    depends_on('gdbm')
+
     # Installing cpanm alongside the core makes it safe and simple for
     # people/projects to install their own sets of perl modules.  Not
     # having it in core increases the "energy of activation" for doing
@@ -81,7 +83,9 @@ def configure_args(self):
 
         config_args = [
             '-des',
-            '-Dprefix={0}'.format(prefix)
+            '-Dprefix={0}'.format(prefix),
+            '-Dlocincpth=' + self.spec['gdbm'].prefix.include,
+            '-Dloclibpth=' + self.spec['gdbm'].prefix.lib
         ]
 
         # Discussion of -fPIC for Intel at: