Option to use system, instead of built-in, expat

At least one Linux distribution packager of FG has complained that we're bundling expat in our binaries. To keep them happy, add the option (-DSYSTEM_EXPAT=1 at cmake time) to use the system expat instead.

For the moment (and simplicity), this requires building SIMGEAR_SHARED (which is fired to on by selecting the option).

simgear/xml/CMakeLists.txt

@@ -7,26 +7,31 @@ set(HEADERS
     )
     
 set(SOURCES 
-    asciitab.h
-    hashtable.h
-    iasciitab.h
-    latin1tab.h
-    nametab.h
-    utf8tab.h
-    xmldef.h
-    xmlparse.h
-    xmlrole.h
-    xmltok.h
-    xmltok_impl.h
-    hashtable.c
-    xmlparse.c
-    xmlrole.c
-    xmltok.c
     easyxml.cxx
-    internal.h
-    ascii.h
-    expat.h
-    expat_external.h
     )
 
+if (NOT SYSTEM_EXPAT)
+    list(APPEND SOURCES
+        asciitab.h
+        hashtable.h
+        iasciitab.h
+        latin1tab.h
+        nametab.h
+        utf8tab.h
+        xmldef.h
+        xmlparse.h
+        xmlrole.h
+        xmltok.h
+        xmltok_impl.h
+        hashtable.c
+        xmlparse.c
+        xmlrole.c
+        xmltok.c
+        internal.h
+        ascii.h
+        expat.h
+        expat_external.h 
+    )
+endif()
+    
 simgear_component(xml xml "${SOURCES}" "${HEADERS}")

simgear/xml/easyxml.cxx

@@ -4,13 +4,22 @@
  * This file is in the Public Domain, and comes with NO WARRANTY of any kind.
  */
 
+#ifdef HAVE_CONFIG_H
+#  include <simgear_config.h>
+#endif
+     
 #include <simgear/compiler.h>
 
 #include <string.h>		// strcmp()
 
 #include "easyxml.hxx"
-#include "xmlparse.h"
-
+     
+#ifdef SYSTEM_EXPAT
+#  include <expat.h>
+#else
+#  include "xmlparse.h"     
+#endif
+     
 #include <fstream>
 #include <iostream>