Versioning for Thrift, remove thrift/lib/cpp/config.h

Summary:
Also, regenerate thrift_config.h.

Move .gitignore to public_tld so I can add thrift_config.h while still
having it checked into fbcode.

Test Plan: fbconfig -r thrift && fbmake runtests_opt

Reviewed By: meyering@fb.com

Subscribers: alandau, bmatheny, jhj, kma, lesha

FB internal diff: D1449068

folly/configure.ac

@@ -10,7 +10,7 @@ AC_INIT([folly], m4_translit(folly_version_str, [:], [.]), [folly@fb.com])
 
 # We assume all revisions are backwards incompatible.
 LT_VERSION=folly_version_str:0
-AC_SUBST(LT_VERSION)
+AC_SUBST([LT_VERSION])
 
 AC_CONFIG_SRCDIR([Likely.h])
 AC_CONFIG_HEADERS([config.h])