bench: fixes for benchmarking harness

This forces the C++ benchmarks that use libc++ to use Clang, which is
apparently the only way it works?

We also disable a benchmark for D's compile time regexes that seems to
either never terminate or take exponential time.

Author: BurntSushi

bench/build.rs

@@ -31,7 +31,9 @@ fn main() {
             cc::Build::new()
                 .cpp(true)
                 .file("src/ffi/stdcpp.cpp")
+                .compiler("clang++")
                 .cpp_link_stdlib("c++")
+                .cpp_set_stdlib("c++")
                 .compile("libcstdcpp.a");
         }
     }

bench/src/sherlock.rs

@@ -178,6 +178,8 @@ sherlock!(holmes_cochar_watson, r"Holmes.{0,25}Watson|Watson.{0,25}Holmes", 7);
 #[cfg(not(feature = "re-stdcpp"))]
 #[cfg(not(feature = "re-boost"))]
 #[cfg(not(feature = "re-tcl"))]
+#[cfg(not(feature = "re-dphobos-dmd-ct"))]
+#[cfg(not(feature = "re-dphobos-ldc-ct"))]
 sherlock!(
     holmes_coword_watson,
     r"Holmes(?:\s*.+\s*){0,10}Watson|Watson(?:\s*.+\s*){0,10}Holmes",