Change link strategy: set rpath instead of linking some libs statically

Hopefully this doesn't break things for people trying non-default
toolchains, but it might.

We were previously using -static-libstdc++ to make sure that we would
find the right libstdc++.  I noticed that we weren't getting the right
version of libgcc_s (using the system one instead of the freshly-built
one), so use rpath for now.

src/Makefile

@@ -161,18 +161,8 @@ COMMON_LDFLAGS += `pkg-config tinfo && pkg-config tinfo --libs || echo ""`
 # TODO should probably do the linking before MCJIT
 COMMON_LDFLAGS += -Wl,-E
 
-# The system libstdc++ will likely be too old for Pyston, but we made a brand new one
-# as part of our GCC installation.  There are a couple ways to use that one instead:
-# 1) Install the built libstdc++ (~/pyston_deps/gcc-4.8.2-install/lib64/libstdc++.so.6.0.18) and update the global shared library (/usr/local/x86_64-linux-gnu/libstdc++.so.6) to point to it
-# 2) Set LD_LIBRARY_PATH=~/pyston_deps/gcc-4.8.2-install/lib64
-# 3) Link it in statically.
-#
-# For now, go with option #3 since it is the only one that requires no extra work for people who want to run Pyston.
-#
-# (I was surprised by this issue: the linker sees the shared library that we built
-# as part of GCC and decides to dynamically link to that, even though at load time we'll
-# end up linking to the system libstdc++ instead.)
-COMMON_LDFLAGS += -static-libstdc++
+# We get multiple shared libraries (libstdc++, libgcc_s) from the gcc installation:
+COMMON_LDFLAGS += -Wl,-rpath $(GCC_DIR)/lib64
 
 ifneq ($(USE_DEBUG_LIBUNWIND),0)
 	COMMON_LDFLAGS += -L$(DEPS_DIR)/libunwind-trunk-debug-install/lib