gb_LinkTarget__add_internal_headers was used only for sdi headers now
anyway, hopefully for no good reason.
One headers target is enough for all.
Change-Id: Ia01fb7a0aac7f72915448424b3393ecd9ba0f8f5
add a new gb_LinkTarget_use_system_win32_libs to abstract different
linker options on MSVC and GCC.
Change-Id: Ic9bf2545f59bf7871e6fc06b290c486ddfbec03d