postgresql: Fix build with threadsafe and add variant. (#4403)

* postgresql: Fix build with threadsafe and add variant.

* postgresql: Convert Package to AutotoolsPackage.

Also add explicit enable flag for +threadsafe variant.

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

@@ -25,7 +25,7 @@
 from spack import *
 
 
-class Postgresql(Package):
+class Postgresql(AutotoolsPackage):
     """PostgreSQL is a powerful, open source object-relational database system.
     It has more than 15 years of active development and a proven architecture
     that has earned it a strong reputation for reliability, data integrity, and
@@ -40,8 +40,13 @@ class Postgresql(Package):
     depends_on('openssl')
     depends_on('readline')
 
-    def install(self, spec, prefix):
-        configure("--prefix=%s" % prefix,
-                  "--with-openssl")
-        make()
-        make("install")
+    variant('threadsafe', default=False, description='Build with thread safe.')
+
+    def configure_arg(self):
+        config_args = ["--with-openssl"]
+        if '+threadsafe' in self.spec:
+            config_args.append("--enable-thread-safety")
+        else:
+            config_args.append("--disable-thread-safety")
+
+        return config_args