Fix compilation with curl <7.21.6 used under CentOS 6

According to cURL documentation, the CURLOPT_ACCEPT_ENCODING symbol
was called CURLOPT_ENCODING earlier.

Closes https://github.com/wxWidgets/wxWidgets/pull/2210

src/common/webrequest_curl.cpp

@@ -37,6 +37,11 @@
     #define wxCURL_HAVE_MULTI_WAIT 0
 #endif
 
+// The new name was introduced in curl 7.21.6.
+#ifndef CURLOPT_ACCEPT_ENCODING
+    #define CURLOPT_ACCEPT_ENCODING CURLOPT_ENCODING
+#endif
+
 //
 // wxWebResponseCURL
 //