Don't demangle Rust symbols by default, but allow linking to rust-demangle.

The Rust compiler uses GCC C++ name mangling, but it has another layer of
encoding so abi::cxa_demangle doesn't produce great results. This patch
changes dump_syms to dump unmangled names by default so that consumers can
demangle them after-the-fact.

It also adds a tiny bit of support for linking against a Rust library I wrote
that can demangle Rust symbols nicely:
https://github.com/luser/rust-demangle-capi

BUG=

Change-Id: I63a425035ebb7ac516f067fed2aa782849ea9604
Reviewed-on: https://chromium-review.googlesource.com/402308
Reviewed-by: Mark Mentovai <mark@chromium.org>

configure

@@ -626,6 +626,8 @@ ac_subst_vars='am__EXEEXT_FALSE
 am__EXEEXT_TRUE
 LTLIBOBJS
 LIBOBJS
+RUST_DEMANGLE_LIBS
+RUST_DEMANGLE_CFLAGS
 SELFTEST_FALSE
 SELFTEST_TRUE
 GTEST_LIBS
@@ -775,6 +777,7 @@ enable_processor
 enable_tools
 enable_system_test_libs
 enable_selftest
+with_rust_demangle
 '
       ac_precious_vars='build_alias
 host_alias
@@ -795,7 +798,9 @@ GMOCK_CFLAGS
 GMOCK_LIBS
 GTEST_CONFIG
 GTEST_CFLAGS
-GTEST_LIBS'
+GTEST_LIBS
+RUST_DEMANGLE_CFLAGS
+RUST_DEMANGLE_LIBS'
 
 
 # Initialize some variables set by options.
@@ -1433,6 +1438,14 @@ Optional Features:
   --enable-selftest       Run extra tests with "make check" (may conflict with
                           optimizations) (default is no)
 
+Optional Packages:
+  --with-PACKAGE[=ARG]    use PACKAGE [ARG=yes]
+  --without-PACKAGE       do not use PACKAGE (same as --with-PACKAGE=no)
+  --with-rust-demangle=/path/to/rust-demangle-capi
+                          Link against the rust-demangle library to demangle
+                          Rust language symbols during symbol dumping (default
+                          is no) Pass the path to the crate root.
+
 Some influential environment variables:
   CC          C compiler command
   CFLAGS      C compiler flags
@@ -1456,6 +1469,10 @@ Some influential environment variables:
   GTEST_CFLAGS
               Compiler flags for gtest
   GTEST_LIBS  Linker flags for gtest
+  RUST_DEMANGLE_CFLAGS
+              Compiler flags for rust-demangle
+  RUST_DEMANGLE_LIBS
+              Linker flags for rust-demangle
 
 Use these variables to override the choices made by `configure' or to help
 it to find libraries and programs with nonstandard names/locations.
@@ -7747,6 +7764,31 @@ else
 fi
 
 
+
+# Check whether --with-rust-demangle was given.
+if test "${with_rust_demangle+set}" = set; then :
+  withval=$with_rust_demangle; case "${withval}" in
+               yes)
+                 as_fn_error $? "You must pass the path to the rust-demangle-capi crate for --with-rust-demangle" "$LINENO" 5
+                 ;;
+               no)
+                 rust_demangle=false
+                 ;;
+               *)
+                 if ! test -f "${withval}/Cargo.toml"; then
+                   as_fn_error $? "You must pass the path to the rust-demangle-capi crate for --with-rust-demangle" "$LINENO" 5
+                 fi
+                 RUST_DEMANGLE_CFLAGS="-DHAVE_RUST_DEMANGLE -I${withval}/target/include"
+                 RUST_DEMANGLE_LIBS="-L${withval}/target/release -lrust_demangle -lpthread -ldl"
+                 ;;
+             esac
+else
+  rust_demangle=false
+fi
+
+
+
+
 ac_config_files="$ac_config_files breakpad.pc breakpad-client.pc Makefile"