Dynamically link libc++, if integrating with system LLVM

This ensures that the zigcpp clang driver and LLVM are using the
same copy of libc++. See prior commit for more details.

Author: topolarity

build.zig

@@ -553,21 +553,21 @@ fn addCmakeCfgOptionsToExe(
         // System -lc++ must be used because in this code path we are attempting to link
         // against system-provided LLVM, Clang, LLD.
         if (exe.target.getOsTag() == .linux) {
-            // First we try to static link against gcc libstdc++. If that doesn't work,
+            // First we try to dynamically link against gcc libstdc++. If that doesn't work,
             // we fall back to -lc++ and cross our fingers.
-            addCxxKnownPath(b, cfg, exe, "libstdc++.a", "", need_cpp_includes) catch |err| switch (err) {
+            addCxxKnownPath(b, cfg, exe, "libstdc++.so", "", need_cpp_includes) catch |err| switch (err) {
                 error.RequiredLibraryNotFound => {
                     exe.linkSystemLibrary("c++");
                 },
                 else => |e| return e,
             };
             exe.linkSystemLibrary("unwind");
         } else if (exe.target.isFreeBSD()) {
-            try addCxxKnownPath(b, cfg, exe, "libc++.a", null, need_cpp_includes);
+            try addCxxKnownPath(b, cfg, exe, "libc++.so", null, need_cpp_includes);
             exe.linkSystemLibrary("pthread");
         } else if (exe.target.getOsTag() == .openbsd) {
-            try addCxxKnownPath(b, cfg, exe, "libc++.a", null, need_cpp_includes);
-            try addCxxKnownPath(b, cfg, exe, "libc++abi.a", null, need_cpp_includes);
+            try addCxxKnownPath(b, cfg, exe, "libc++.so", null, need_cpp_includes);
+            try addCxxKnownPath(b, cfg, exe, "libc++abi.so", null, need_cpp_includes);
         } else if (exe.target.isDarwin()) {
             exe.linkSystemLibrary("c++");
         }