Added a check for OpenSceneGraph and created some basic documentation.

Makefile.am

@@ -6,6 +6,7 @@ EXTRA_DIST = \
 	README.zlib \
 	README.plib \
 	README.OpenAL \
+	README.OSG \
 	projects \
 	SimGear.dsp \
 	SimGear.dsw

README.OSG

@@ -0,0 +1,24 @@
+[This file is mirrored in both the FlightGear and SimGear packages.]
+
+You *must* have OpenSceneGraph (OSG) installed to build this version of 
+FlightGear.
+
+You can get the latest version of OSG from:
+
+http://www.openscenegraph.org/
+
+Build notes:
+
+Unzip the file OpenSceneGraph-2.0.zip and install using the following
+commands:
+
+unzip OpenSceneGraph-2.0
+cd OpenSceneGraph
+ccmake .
+
+[ While running ccmake: press 'c' to configure, press 'c' once more, and
+  then press 'g' to generate and exit ]
+
+make 
+sudo make install
+

configure.ac

@@ -386,6 +386,18 @@ int main() {
 
 LIBS="$saved_LIBS"
 
+AC_CHECK_HEADER(osg/Version)
+if test "x$ac_cv_header_osg_Version" != "xyes"; then
+    echo
+    echo "You *must* have the OpenSceneGraph support library installed on your system"
+    echo "to build the FGFS simulator!"
+    echo
+    echo "Please see README.OSG for more details."
+    echo
+    echo "configure aborted."
+    exit
+fi
+
 AC_LANG_POP
 
 dnl Check for system installed zlib