Clang attempts to guess file names based on the OS and distro (yes!),
but unfortunately, that doesn't work for us.

This patch makes it easy to insert libc's $libdir so that Clang passes the
correct absolute file name of crt1.o etc. to 'ld'.  It also disables all
the distro-specific stuff and removes the hard-coded FHS directory names
to make sure Clang also works on foreign distros.

diff --git a/lib/Driver/Distro.cpp b/lib/Driver/Distro.cpp
index ee4fe841..f0313bbe 100644
--- a/lib/Driver/Distro.cpp
+++ b/lib/Driver/Distro.cpp
@@ -93,6 +93,10 @@ static Distro::DistroType DetectLsbRelease(llvm::vfs::FileSystem &VFS) {
 }
 
 static Distro::DistroType DetectDistro(llvm::vfs::FileSystem &VFS) {
+  // The compiler should always behave the same, even when used via Guix on a
+  // foreign distro.
+  return Distro::UnknownDistro;
+
   Distro::DistroType Version = Distro::UnknownDistro;
 
   // Newer freedesktop.org's compilant systemd-based systems
diff --git a/lib/Driver/ToolChains/Cuda.cpp b/lib/Driver/ToolChains/Cuda.cpp
index d14776c5..88bc3ccd 100644
--- a/lib/Driver/ToolChains/Cuda.cpp
+++ b/lib/Driver/ToolChains/Cuda.cpp
@@ -119,6 +119,9 @@ CudaInstallationDetector::CudaInstallationDetector(
     const Driver &D, const llvm::Triple &HostTriple,
     const llvm::opt::ArgList &Args)
     : D(D) {
+  // Don't look for CUDA in /usr.
+  return;
+
   struct Candidate {
     std::string Path;
     bool StrictChecking;
diff --git a/lib/Driver/ToolChains/Linux.cpp b/lib/Driver/ToolChains/Linux.cpp
--- a/lib/Driver/ToolChains/Linux.cpp
+++ b/lib/Driver/ToolChains/Linux.cpp
@@ -186,6 +186,10 @@
 
   Generic_GCC::PushPPaths(PPaths);
 
+  // Comment out the distro-specific tweaks so that they don't bite when
+  // using Guix on a foreign distro.
+#if 0
+
   Distro Distro(D.getVFS(), Triple);
 
   if (Distro.IsAlpineLinux() || Triple.isAndroid()) {
@@ -251,6 +255,7 @@
 
   if (IsAndroid || Distro.IsOpenSUSE())
     ExtraOpts.push_back("--enable-new-dtags");
+#endif
 
   // The selection of paths to try here is designed to match the patterns which
   // the GCC driver itself uses, as this is part of the GCC-compatible driver.
@@ -264,6 +269,7 @@
 
   Generic_GCC::AddMultilibPaths(D, SysRoot, OSLibDir, MultiarchTriple, Paths);
 
+#if 0
   addPathIfExists(D, SysRoot + "/lib/" + MultiarchTriple, Paths);
   addPathIfExists(D, SysRoot + "/lib/../" + OSLibDir, Paths);
 
@@ -295,9 +301,11 @@
     addPathIfExists(D, SysRoot + "/" + OSLibDir + "/" + ABIName, Paths);
     addPathIfExists(D, SysRoot + "/usr/" + OSLibDir + "/" + ABIName, Paths);
   }
+#endif
 
   Generic_GCC::AddMultiarchPaths(D, SysRoot, OSLibDir, Paths);
 
+#if 0
   // Similar to the logic for GCC above, if we are currently running Clang
   // inside of the requested system root, add its parent library path to those
   // searched.
@@ -305,9 +313,14 @@
   // directory ('Dir' below) or the ResourceDir.
   if (StringRef(D.Dir).startswith(SysRoot))
     addPathIfExists(D, D.Dir + "/../lib", Paths);
+#endif
+
+  // Add libc's lib/ directory to the search path, so that crt1.o, crti.o,
+  // and friends can be found.
+  addPathIfExists(D, "@GLIBC_LIBDIR@", Paths);
 
-  addPathIfExists(D, SysRoot + "/lib", Paths);
-  addPathIfExists(D, SysRoot + "/usr/lib", Paths);
+  // Add GCC's lib/ directory so libstdc++.so can be found.
+  addPathIfExists(D, GCCInstallation.getParentLibPath(), Paths);
 }
 
 ToolChain::RuntimeLibType Linux::GetDefaultRuntimeLibType() const {
'>2023-09-24</span></td><td><a href='/guix/commit/etc/committer.scm.in?id=0836af9a3b33813e69ece7075fbc222fdb85bfb3'>etc/committer: Do not recompute changes when there are no definitions.</a><span class='msg-avail'>...</span></td><td>Ricardo Wurmus</td></tr>
<tr><td><span title='2022-05-31 14:52:13 -0400'>2022-05-31</span></td><td><a href='/guix/commit/etc/committer.scm.in?id=5e6efdfeecae82315862ebee95ce02bb868eb0f3'>etc/committer: Teach it how to commit package removal.</a><span class='msg-avail'>...</span></td><td>Maxim Cournoyer</td></tr>
<tr><td><span title='2022-05-31 14:52:13 -0400'>2022-05-31</span></td><td><a href='/guix/commit/etc/committer.scm.in?id=6c956243bc7972644e947563a45e33d73042afdc'>etc/committer: Prefix (sxml xpath) symbols to avoid name conflict.</a><span class='msg-avail'>...</span></td><td>Maxim Cournoyer</td></tr>
<tr><td><span title='2022-01-23 14:01:30 +0100'>2022-01-23</span></td><td><a href='/guix/commit/etc/committer.scm.in?id=67d74daf0c8b7b6d786859f0be659021f0a153d8'>etc: committer: Support plain input lists.</a><span class='msg-avail'>...</span></td><td>Ricardo Wurmus</td></tr>
<tr><td><span title='2021-09-21 15:15:52 +0200'>2021-09-21</span></td><td><a href='/guix/commit/etc/committer.scm.in?id=3d15a19874f63dfc15eb6625558f279ccbc71d5d'>etc: committer: Amend previous commit if a copyright line was added.</a><span class='msg-avail'>...</span></td><td>Xinglu Chen</td></tr>
<tr><td><span title='2021-08-10 17:32:19 +0200'>2021-08-10</span></td><td><a href='/guix/commit/etc/committer.scm.in?id=80fc776a614befdae7b2d22f8b154a74e5e679c4'>etc/committer: Pass command-line arguments to main.</a><span class='msg-avail'>...</span></td><td>Sarah Morgensen</td></tr>
<tr><td><span title='2021-08-10 17:32:19 +0200'>2021-08-10</span></td><td><a href='/guix/commit/etc/committer.scm.in?id=73177859bc271e65c7d3c0a44dd7d5103ed8a6db'>etc/committer: Support custom commit messages.</a><span class='msg-avail'>...</span></td><td>Sarah Morgensen</td></tr>
<tr><td><span title='2021-08-07 14:01:20 +0200'>2021-08-07</span></td><td><a href='/guix/commit/etc/committer.scm.in?id=50c2dcd1c977f98681a4e457b2bcf09d96588eee'>etc: committer: Support reading G-expressions.</a><span class='msg-avail'>...</span></td><td>Maxime Devos</td></tr>
<tr><td><span title='2021-05-04 11:52:23 +0200'>2021-05-04</span></td><td><a href='/guix/commit/etc/committer.scm.in?id=570b3d32b92fb2220c5ecd9302f4fa85947d4bff'>etc: Break long lines in commit messages.</a><span class='msg-avail'>...</span></td><td>Ricardo Wurmus</td></tr>