Pass libstdc++.so path to linker under OpenBSD

By default, the linker in use under OpenBSD is the linker of base, which
don't include /usr/local/lib where libstdc++ of gcc-4.9 lives. We need
to add this directory to linker-path-search (using -L).

Search the path of libstdc++.a, which is a known name (libstdc++.so has
SO_VERSION) in the same directory.

mk/platform.mk

@@ -208,6 +208,14 @@ define CFG_MAKE_TOOLCHAIN
 
   ifeq ($$(findstring $(HOST_$(1)),arm aarch64 mips mipsel powerpc),)
 
+  # On OpenBSD, we need to pass the path of libstdc++.so to the linker
+  # (use path of libstdc++.a which is a known name for the same path)
+  ifeq ($(OSTYPE_$(1)),unknown-openbsd)
+    RUSTC_FLAGS_$(1)=-L "$$(dir $$(shell $$(CC_$(1)) $$(CFG_GCCISH_CFLAGS_$(1)) \
+        -print-file-name=lib$(CFG_STDCPP_NAME).a))" \
+        $(RUSTC_FLAGS_$(1))
+  endif
+
   # On Bitrig, we need the relocation model to be PIC for everything
   ifeq (,$(filter $(OSTYPE_$(1)),bitrig))
     LLVM_MC_RELOCATION_MODEL="pic"