Implemented --enable-hidden-symbols configure option to enable

-fvisibility=hidden on gcc >= 4.0.  This reduces the size of the libcurl
binary and speeds up dynamic linking by hiding all the internal symbols from
the symbol table.

include/curl/curl.h

@@ -58,7 +58,17 @@ extern "C" {
 #define CURL_EXTERN  __declspec(dllimport)
 #endif
 #else
-#define CURL_EXTERN
+
+#ifdef CURL_HIDDEN_SYMBOLS
+/*
+ * On gcc >= 4 if -fvisibility=hidden is given then this is used to cause
+ * external definitions to be put into the shared library.  It makes no
+ * difference to applications whether this is set or not, only the library.
+ */
+#define CURL_EXTERN __attribute__ ((visibility ("default")))
+#else
+#define CURL_EXTERN 
+#endif
 #endif
 
 /*