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.

--- cfe-3.6.0.src/lib/Driver/ToolChains.cpp	2015-02-18 22:03:07.000000000 +0100
+++ cfe-3.6.0.src/lib/Driver/ToolChains.cpp	2015-06-19 16:37:20.459701044 +0200
@@ -2931,6 +2931,9 @@ Linux::Linux(const Driver &D, const llvm
 
   Linker = GetLinkerPath();
 
+  // Comment out the distro-specific tweaks so that they don't bite when
+  // using Guix on a foreign distro.
+#if 0
   Distro Distro = DetectDistro(Arch);
 
   if (IsOpenSUSE(Distro) || IsUbuntu(Distro)) {
@@ -2973,6 +2976,7 @@ Linux::Linux(const Driver &D, const llvm
 
   if (IsOpenSUSE(Distro))
     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.
@@ -3043,14 +3047,12 @@ Linux::Linux(const Driver &D, const llvm
     addPathIfExists(D.Dir + "/../" + OSLibDir, Paths);
   }
 
-  addPathIfExists(SysRoot + "/lib/" + MultiarchTriple, Paths);
-  addPathIfExists(SysRoot + "/lib/../" + OSLibDir, Paths);
-  addPathIfExists(SysRoot + "/usr/lib/" + MultiarchTriple, Paths);
-  addPathIfExists(SysRoot + "/usr/lib/../" + OSLibDir, Paths);
-
   // 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(SysRoot + "/usr/lib/" + GCCInstallation.getTriple().str() +
                     "/../../" + OSLibDir, Paths);
 
@@ -3060,6 +3062,7 @@ Linux::Linux(const Driver &D, const llvm
       addPathIfExists(GCCInstallation.getInstallPath() +
                       BiarchSibling.gccSuffix(), Paths);
     }
+#endif
 
     // See comments above on the multilib variant for details of why this is
     // included even from outside the sysroot.
@@ -3083,8 +3086,9 @@ Linux::Linux(const Driver &D, const llvm
   if (StringRef(D.Dir).startswith(SysRoot))
     addPathIfExists(D.Dir + "/../lib", Paths);
 
-  addPathIfExists(SysRoot + "/lib", Paths);
-  addPathIfExists(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("@GLIBC_LIBDIR@", Paths);
 }
 
 bool Linux::HasNativeLLVMSupport() const {