configure: add symbols versioning option

Allow, at configure time, the production of versioned symbols. The
symbols will look like "CURL_<FLAVOUR>_<VERSION> <SYMBOL>", where
<FLAVOUR> represents the SSL flavour (e.g. OPENSSL, GNUTLS, NSS, ...),
<VERSION> is the major SONAME version and <SYMBOL> is the actual symbol
name. If no SSL library is enabled the symbols will be just
"CURL_<VERSION> <SYMBOL>".

lib/libcurl.vers.in

@@ -0,0 +1,13 @@
+HIDDEN
+{
+  local:
+    __*;
+    _rest*;
+    _save*;
+};
+
+CURL_@VERSIONED_FLAVOUR@4
+{
+  global: curl_*;
+  local: *;
+};