stage3 bsd: support dynamic libstdc++/libc++

Currently llvm-linkage mode (static vs dynamic) decides linkage mode
for system libstdc++/libc++ .

A previous commit only tested static mode for *BSD and netbsd was
reported to not work in dynamic mode.

We now special-case freebsd, openbsd, netbsd and dragonfly for dynamic
linking too.

Author: mikdusan

build.zig

@@ -556,14 +556,11 @@ fn addCmakeCfgOptionsToExe(
     if (use_zig_libcxx) {
         exe.linkLibCpp();
     } else {
-        const need_cpp_includes = true;
-        const lib_suffix = switch (cfg.llvm_linkage) {
-            .static => exe.target.staticLibSuffix()[1..],
-            .dynamic => exe.target.dynamicLibSuffix()[1..],
-        };
-
         // System -lc++ must be used because in this code path we are attempting to link
         // against system-provided LLVM, Clang, LLD.
+        const need_cpp_includes = true;
+        const static = cfg.llvm_linkage == .static;
+        const lib_suffix = if (static) exe.target.staticLibSuffix()[1..] else exe.target.dynamicLibSuffix()[1..];
         switch (exe.target.getOsTag()) {
             .linux => {
                 // First we try to link against gcc libstdc++. If that doesn't work, we fall
@@ -580,20 +577,24 @@ fn addCmakeCfgOptionsToExe(
                 exe.linkSystemLibrary("c++");
             },
             .freebsd => {
-                try addCxxKnownPath(b, cfg, exe, b.fmt("libc++.{s}", .{lib_suffix}), null, need_cpp_includes);
-                try addCxxKnownPath(b, cfg, exe, b.fmt("libgcc_eh.{s}", .{lib_suffix}), null, need_cpp_includes);
+                if (static) {
+                    try addCxxKnownPath(b, cfg, exe, b.fmt("libc++.{s}", .{lib_suffix}), null, need_cpp_includes);
+                    try addCxxKnownPath(b, cfg, exe, b.fmt("libgcc_eh.{s}", .{lib_suffix}), null, need_cpp_includes);
+                } else {
+                    try addCxxKnownPath(b, cfg, exe, b.fmt("libc++.{s}", .{lib_suffix}), null, need_cpp_includes);
+                }
             },
             .openbsd => {
                 try addCxxKnownPath(b, cfg, exe, b.fmt("libc++.{s}", .{lib_suffix}), null, need_cpp_includes);
                 try addCxxKnownPath(b, cfg, exe, b.fmt("libc++abi.{s}", .{lib_suffix}), null, need_cpp_includes);
             },
-            .netbsd => {
-                try addCxxKnownPath(b, cfg, exe, b.fmt("libstdc++.{s}", .{lib_suffix}), null, need_cpp_includes);
-                try addCxxKnownPath(b, cfg, exe, b.fmt("libgcc_eh.{s}", .{lib_suffix}), null, need_cpp_includes);
-            },
-            .dragonfly => {
-                try addCxxKnownPath(b, cfg, exe, b.fmt("libstdc++.{s}", .{lib_suffix}), null, need_cpp_includes);
-                try addCxxKnownPath(b, cfg, exe, b.fmt("libgcc_eh.{s}", .{lib_suffix}), null, need_cpp_includes);
+            .netbsd, .dragonfly => {
+                if (static) {
+                    try addCxxKnownPath(b, cfg, exe, b.fmt("libstdc++.{s}", .{lib_suffix}), null, need_cpp_includes);
+                    try addCxxKnownPath(b, cfg, exe, b.fmt("libgcc_eh.{s}", .{lib_suffix}), null, need_cpp_includes);
+                } else {
+                    try addCxxKnownPath(b, cfg, exe, b.fmt("libstdc++.{s}", .{lib_suffix}), null, need_cpp_includes);
+                }
             },
             else => {},
         }