When linking mostly-static Linux binaries, link libstdc++.a explicitly.

This allows libstdc++ to be statically linked, which is normally only
possible when invoking GCC as `g++` with the `-static-libstdc++` flag.

Fixes bazelbuild#2840

See envoyproxy/envoy#415 for additional
background and context.

tools/cpp/CROSSTOOL.tpl

@@ -115,6 +115,7 @@ toolchain {
 %{opt_content}
   }
   linking_mode_flags { mode: DYNAMIC }
+%{link_content}
 
 %{coverage}
 }

tools/cpp/unix_cc_configure.bzl

@@ -46,6 +46,24 @@ def _build_crosstool(d, prefix="  "):
   return "\n".join(lines)
 
 
+def _build_link_content():
+  # `-static-libstdc++` is only supported when invoking GCC as `g++`, and
+  # `-lstdc++` forces dynamic linking of libstdc++. To get desired
+  # mostly-static behavior, invoke the link by explicitly naming a static
+  # library archive.
+  #
+  # https://github.com/bazelbuild/bazel/issues/2840
+  return """
+  linking_mode_flags {
+    mode: DYNAMIC
+    linker_flag: "-lstdc++"
+  }
+  linking_mode_flags {
+    mode: MOSTLY_STATIC
+    linker_flag: "-l:libstdc++.a"
+  }"""
+
+
 def _build_tool_path(d):
   """Build the list of %-escaped tool_path for the CROSSTOOL file."""
   lines = []
@@ -174,7 +192,6 @@ def _crosstool_content(repository_ctx, cc, cpu_value, darwin):
           "-std=c++0x",
       ] + _escaped_cplus_include_paths(repository_ctx),
       "linker_flag": [
-          "-lstdc++",
           "-lm",  # Some systems expect -lm in addition to -lstdc++
           # Anticipated future default.
       ] + (
@@ -385,6 +402,7 @@ def configure_unix_toolchain(repository_ctx, cpu_value):
                     _build_tool_path(tool_paths),
       "%{opt_content}": _build_crosstool(opt_content, "    "),
       "%{dbg_content}": _build_crosstool(dbg_content, "    "),
+      "%{link_content}": _build_link_content(),
       "%{cxx_builtin_include_directory}": "",
       "%{coverage}": _coverage_feature(darwin),
       "%{msvc_env_tmp}": "",

tools/cpp/windows_cc_configure.bzl

@@ -306,6 +306,7 @@ def configure_windows_toolchain(repository_ctx):
         "%{content}": _get_escaped_windows_msys_crosstool_content(repository_ctx),
         "%{opt_content}": "",
         "%{dbg_content}": "",
+        "%{link_content}": "",
         "%{cxx_builtin_include_directory}": "",
         "%{coverage}": "",
     })