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 2c4d44f..0493b0a 100644
--- a/lib/Driver/Distro.cpp
+++ b/lib/Driver/Distro.cpp
@@ -18,6 +18,10 @@ using namespace clang::driver;
 using namespace clang;
 
 static Distro::DistroType DetectDistro(vfs::FileSystem &VFS) {
+  // The compiler should always behave the same, even when used via Guix on a
+  // foreign distro.
+  return Distro::UnknownDistro;
+
   llvm::ErrorOr<std::unique_ptr<llvm::MemoryBuffer>> File =
       VFS.getBufferForFile("/etc/lsb-release");
   if (File) {
diff --git a/lib/Driver/ToolChains/Cuda.cpp b/lib/Driver/ToolChains/Cuda.cpp
index 7fb4ae4..cb21344 100644
--- a/lib/Driver/ToolChains/Cuda.cpp
+++ b/lib/Driver/ToolChains/Cuda.cpp
@@ -66,6 +66,9 @@ 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
index f8f3623..36dc722 100644
--- a/lib/Driver/ToolChains/Linux.cpp
+++ b/lib/Driver/ToolChains/Linux.cpp
@@ -225,7 +225,9 @@ Linux::Linux(const Driver &D, const llvm::Triple &Triple, const ArgList &Args)
   PPaths.push_back(Twine(GCCInstallation.getParentLibPath() + "/../" +
                          GCCInstallation.getTriple().str() + "/bin")
                        .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()) {
@@ -284,6 +286,7 @@ 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.
@@ -348,6 +351,7 @@ Linux::Linux(const Driver &D, const llvm::Triple &Triple, const ArgList &Args)
     }
   }
 
+#if 0
   // Similar to the logic for GCC above, if we currently running Clang inside
   // of the requested system root, add its parent library paths to
   // those searched.
@@ -393,6 +397,8 @@ Linux::Linux(const Driver &D, const llvm::Triple &Triple, const ArgList &Args)
   // 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.
     addPathIfExists(D,
                     SysRoot + "/usr/lib/" + GCCInstallation.getTriple().str() +
                         "/../../" + OSLibDir,
@@ -428,9 +434,14 @@ Linux::Linux(const Driver &D, const llvm::Triple &Triple, const ArgList &Args)
   // 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);
 }
 
 bool Linux::HasNativeLLVMSupport() const { return true; }
mentation.scm?id=189be331acfda1c242a9c85fca8d2a0356742f48'>build-system/gnu: Add 'bootstrap' phase.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2018-02-09 01:46:34 -0500'>2018-02-09</span></td><td><a href='/guix/commit/gnu/packages/documentation.scm?id=efe2a2833c6d306d0c60127fdfebaff6dc415b4c'>Merge branch 'master' into core-updates</a></td><td>Mark H Weaver</td></tr>
<tr><td><span title='2018-02-06 20:02:12 +0100'>2018-02-06</span></td><td><a href='/guix/commit/gnu/packages/documentation.scm?id=ccfe4aa1c09b5205a1d17070eb4f9f6da59650bc'>gnu: asciidoc: Update to 8.6.10.</a><span class='msg-avail'>...</span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2017-11-11 17:28:51 +0200'>2017-11-11</span></td><td><a href='/guix/commit/gnu/packages/documentation.scm?id=738d0cd665ef89a77af0b1568aea68d29bc749d6'>gnu: doxygen: Build with flex@2.6.1.</a><span class='msg-avail'>...</span></td><td>Efraim Flashner</td></tr>
<tr><td><span title='2017-08-29 20:13:24 -0400'>2017-08-29</span></td><td><a href='/guix/commit/gnu/packages/documentation.scm?id=3c8ba11a97d3f3b671b8919da8c41f0b0e2ccaf3'>Update e-mail for Kei Kebreau.</a><span class='msg-avail'>...</span></td><td>Kei Kebreau</td></tr>
<tr><td><span title='2017-06-20 18:10:35 +0530'>2017-06-20</span></td><td><a href='/guix/commit/gnu/packages/documentation.scm?id=148585c240f848ad08f6541ede7db3fca3411007'>gnu: Move contents of zip module into compression module.</a><span class='msg-avail'>...</span></td><td>Arun Isaac</td></tr>