Bake dependency runtime library_dirs into binary RUNPATH; default Linux toolchain to glibc

R2: flags.cppm emitted -Wl,-rpath only from toolchain.linkRuntimeDirs, dropping
dependency packages' [runtime] library_dirs (already collected into
plan.runtimeLibraryDirs by plan.cppm). So host-GL passthrough dirs (e.g.
compat.glx-runtime) never reached the binary RUNPATH and dlopen()'d libGL/libGLX
failed at run time (GLX: Failed to load GLX). Iterate plan.runtimeLibraryDirs
(superset) instead.

R1: cli.cppm first-run default on Linux was gcc-musl-static, which cannot link
the glibc world (X11/GL/system libs). Default to platform-native glibc gcc;
musl-static stays opt-in via --target x86_64-linux-musl (Cargo-style).

Author: d2learn-org

src/build/flags.cppm

@@ -256,7 +256,13 @@ CompileFlags compute_flags(const BuildPlan& plan) {
     std::string static_stdlib = (f.staticStdlib && !isClang && !mcpp::platform::is_windows) ? " -static-libstdc++" : "";
     std::string runtime_dirs;
     if constexpr (mcpp::platform::supports_rpath) {
-        for (auto& dir : plan.toolchain.linkRuntimeDirs) {
+        // Bake ALL resolved runtime library dirs into the binary's RUNPATH —
+        // not just the toolchain's. plan.runtimeLibraryDirs is the union of
+        // dependency packages' [runtime] library_dirs (e.g. compat.glx-runtime's
+        // host-GL passthrough dir) plus the toolchain/payload dirs. Using only
+        // toolchain.linkRuntimeDirs here dropped dependency runtime dirs, so
+        // dlopen()'d host libs (libGL/libGLX) were unreachable at run time.
+        for (auto& dir : plan.runtimeLibraryDirs) {
             runtime_dirs += " -L" + escape_path(dir);
             runtime_dirs += " -Wl,-rpath," + escape_path(dir);
         }

src/cli.cppm

@@ -1386,9 +1386,13 @@ prepare_build(bool print_fingerprint,
         //
         // macOS: LLVM/Clang — Apple doesn't ship GCC; upstream LLVM with
         //        bundled libc++ is the self-contained choice.
-        // Linux: musl-gcc — produces portable static binaries.
+        // Linux: glibc gcc — the platform-native ABI. A musl-static default
+        //        cannot link the glibc world (X11/GL/system libs), so it
+        //        breaks GUI/native packages out of the box. musl-static stays
+        //        opt-in via `mcpp build --target x86_64-linux-musl` for users
+        //        who explicitly want portable static binaries.
         std::string defaultSpec = (mcpp::platform::is_macos || mcpp::platform::is_windows)
-            ? "llvm@20.1.7" : "gcc@15.1.0-musl";
+            ? "llvm@20.1.7" : "gcc@16.1.0";
         auto defaultParsed = mcpp::toolchain::parse_toolchain_spec(defaultSpec);
         auto defaultPkg = mcpp::toolchain::to_xim_package(*defaultParsed);
 
@@ -1398,7 +1402,7 @@ prepare_build(bool print_fingerprint,
                             defaultSpec));
         } else {
             mcpp::ui::info("First run",
-                std::format("no toolchain configured — installing {} (musl, static) as default",
+                std::format("no toolchain configured — installing {} (glibc, native ABI) as default",
                             defaultSpec));
         }