Fix undefined symbols linker error on linux / relwithdebinfo.

See http://gcc.gnu.org/onlinedocs/gcc/C_002b_002b-Interface.html
With this patch, the #pragma will also be used with icc and clang,
I didn't test this.

indra/llcommon/llthread.cpp

@@ -29,6 +29,11 @@
  * $/LicenseInfo$
  */
 
+#ifdef __GNUC__
+// Generate code for inlines from llthread.h (needed for is_main_thread()).
+#pragma implementation "llthread.h"
+#endif
+
 #include "linden_common.h"
 #include "llapr.h"