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 non-GuixSD systems.

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
@@ -240,6 +240,9 @@ Linux::Linux(const Driver &D, const llvm::Triple &Triple, const ArgList &Args)
                          .str());
   }
 
+// 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());
 
   if (Distro.IsAlpineLinux() || Triple.isAndroid()) {
@@ -306,6 +309,7 @@ Linux::Linux(const Driver &D, const llvm::Triple &Triple, const ArgList &Args)
 
   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.
@@ -363,7 +367,7 @@ Linux::Linux(const Driver &D, const llvm::Triple &Triple, const ArgList &Args)
     // the cross. Note that GCC does include some of these directories in some
     // configurations but this seems somewhere between questionable and simply
     // a bug.
-    if (StringRef(LibPath).startswith(SysRoot)) {
+    if (0) {
       addPathIfExists(D, LibPath + "/" + MultiarchTriple, Paths);
       addPathIfExists(D, LibPath + "/../" + OSLibDir, Paths);
     }
@@ -382,6 +386,8 @@ Linux::Linux(const Driver &D, const llvm::Triple &Triple, const ArgList &Args)
   addPathIfExists(D, SysRoot + "/lib/" + MultiarchTriple, Paths);
   addPathIfExists(D, SysRoot + "/lib/../" + OSLibDir, Paths);
 
+// This requires the commented distro tweaks above.
+#if 0
   if (IsAndroid) {
     // Android sysroots contain a library directory for each supported OS
     // version as well as some unversioned libraries in the usual multiarch
@@ -410,10 +416,15 @@ Linux::Linux(const Driver &D, const llvm::Triple &Triple, const ArgList &Args)
     addPathIfExists(D, SysRoot + "/" + OSLibDir + "/" + ABIName, Paths);
     addPathIfExists(D, SysRoot + "/usr/" + OSLibDir + "/" + ABIName, Paths);
   }
+#endif
 
   // Try walking via the GCC triple path in case of biarch or multiarch GCC
   // installations with strange symlinks.
   if (GCCInstallation.isValid()) {
+
+// The following code would end up adding things like
+// "/usr/lib/x86_64-unknown-linux-gnu/../../lib64" to the search path.
+#if 0
     addPathIfExists(D,
                     SysRoot + "/usr/lib/" + GCCInstallation.getTriple().str() +
                         "/../../" + OSLibDir,
@@ -426,6 +437,7 @@ Linux::Linux(const Driver &D, const llvm::Triple &Triple, const ArgList &Args)
                              BiarchSibling.gccSuffix(),
                       Paths);
     }
+#endif
 
     // See comments above on the multilib variant for details of why this is
     // included even from outside the sysroot.
@@ -450,8 +462,9 @@ Linux::Linux(const Driver &D, const llvm::Triple &Triple, const ArgList &Args)
   if (StringRef(D.Dir).startswith(SysRoot))
     addPathIfExists(D, D.Dir + "/../lib", Paths);
 
-  addPathIfExists(D, SysRoot + "/lib", Paths);
-  addPathIfExists(D, SysRoot + "/usr/lib", Paths);
+  // 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);
 }
 
 ToolChain::CXXStdlibType Linux::GetDefaultCXXStdlibType() const {
-25</span></td><td><a href='/guix/commit/etc/guix-daemon.cil.in?id=d64e0261d0007413a795c4cf01f9d06b170da3f5'>etc: Add more SELinux permissions for the daemon.</a><span class='msg-avail'>...</span></td><td>Marius Bakke</td></tr>
<tr><td><span title='2020-11-15 16:08:47 +0100'>2020-11-15</span></td><td><a href='/guix/commit/etc/guix-daemon.cil.in?id=62343288ef6dc56027d268ef773ae699a4bbb76d'>etc: Updates for the guix-daemon SELinux policy.</a><span class='msg-avail'>...</span></td><td>Daniel Brooks</td></tr>
<tr><td><span title='2019-09-08 11:49:25 +0200'>2019-09-08</span></td><td><a href='/guix/commit/etc/guix-daemon.cil.in?id=cc98b00857e29074de96a6ed60e325cdfffaea1a'>etc: Remove references to libexec/guix* from SELinux policy.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>