Add an OpenGL extension query function which should be cross platform

simgear/screen/Makefile.am

@@ -19,6 +19,7 @@ include_HEADERS = \
 	texture.hxx \
 	$(IMAGE_SERVER_INCL) \
 	screen-dump.hxx \
+	extensions.hxx \
 	tr.h
 
 libsgscreen_a_SOURCES = \

simgear/screen/extensions.hxx

@@ -0,0 +1,31 @@
+#ifndef __SG_EXTENSIONS_HXX
+#define __SG_EXTENSIONS_HXX 1
+
+#ifndef WIN32
+#include <dlfcn.h>
+#endif
+
+inline void (*SGLookupFunction(const char *func))() {
+
+#if defined( WIN32 )
+        return (void (*)()) wglGetProcAddress(func);
+
+#else
+
+  // If the target system s UNIX and the ARB_get_proc_address
+  // GLX extension is *not* guaranteed to be supported. An alternative
+  // dlsym-based approach will be used instead.
+  #if defined( linux ) || defined ( sgi )
+        void *libHandle;
+        void (*fptr)();
+        libHandle = dlopen("libGL.so", RTLD_LAZY);
+        fptr = (void (*)()) dlsym(libHandle, func);
+        dlclose(libHandle);
+        return fptr;
+  #else
+        return glXGetProcAddressARB(func);
+  #endif
+#endif
+}
+
+#endif