add gb_LinkTarget_get_layer, gb_LinkTarget_LAYER, gb_LinkTarget_get_rpath

extend $SOLARINC with $JDKINCS.
introduce INCLUDE_STL, and do not include it for C files.
replace $() command expansion with `` (Solaris /bin/sh does only backquote).
fix gb_ResTarget_get_target: make target directory first.

Check for mdds >= 0.5.0 as sc now needs it...