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.

This patch makes slight adjustments over "clang-libc-search-path.patch" for
changes in clang 3.8.

--- cfe-3.8.0.src/lib/Driver/ToolChains.cpp
+++ cfe-3.8.0.src/lib/Driver/ToolChains.cpp
@@ -3661,6 +3661,9 @@
                          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 = DetectDistro(D, Arch);
 
   if (IsOpenSUSE(Distro) || IsUbuntu(Distro)) {
@@ -3702,6 +3705,7 @@
 
   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.
@@ -3771,14 +3775,12 @@
     addPathIfExists(D, D.Dir + "/../" + OSLibDir, Paths);
   }
 
-  addPathIfExists(D, SysRoot + "/lib/" + MultiarchTriple, Paths);
-  addPathIfExists(D, SysRoot + "/lib/../" + OSLibDir, Paths);
-  addPathIfExists(D, SysRoot + "/usr/lib/" + MultiarchTriple, Paths);
-  addPathIfExists(D, 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(D,
                     SysRoot + "/usr/lib/" + GCCInstallation.getTriple().str() +
                         "/../../" + OSLibDir,
@@ -3791,6 +3793,7 @@
                              BiarchSibling.gccSuffix(),
                       Paths);
     }
+#endif
 
     // See comments above on the multilib variant for details of why this is
     // included even from outside the sysroot.
@@ -3815,8 +3818,9 @@
   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);
 }
 
 bool Linux::HasNativeLLVMSupport() const { return true; }
17-04-21</span></td><td><a href='/guix/commit/gnu/packages/upnp.scm?id=8a6f168c48d02d494388d10144f6155899a61502'>gnu: miniupnpc: Use @dfn mark-up.</a><span class='msg-avail'>...</span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2017-04-21 14:46:15 +0200'>2017-04-21</span></td><td><a href='/guix/commit/gnu/packages/upnp.scm?id=05b1feb92ad09601c9e9e022aab541113c8b2b53'>gnu: miniupnpc: Update to 2.0.20170421.</a><span class='msg-avail'>...</span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2017-01-12 17:44:28 +0100'>2017-01-12</span></td><td><a href='/guix/commit/gnu/packages/upnp.scm?id=d9cf11e0f00c689f43560ecb893b6cf22c48b2e0'>gnu: miniupnpc: Update to 2.0.20161216.</a><span class='msg-avail'>...</span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2016-11-25 02:21:51 +0100'>2016-11-25</span></td><td><a href='/guix/commit/gnu/packages/upnp.scm?id=e0f8a520eda6b86a71f99eddf2c6968a95c8de0e'>gnu: miniupnpc: Improve synopsis and description.</a><span class='msg-avail'>...</span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2016-11-25 02:21:51 +0100'>2016-11-25</span></td><td><a href='/guix/commit/gnu/packages/upnp.scm?id=5af82630feb1c6b65edc3dd14bf05d025b77b209'>gnu: miniupnpc: Use the correct ‘upnpc’ in ‘external-ip’.</a><span class='msg-avail'>...</span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2016-11-25 02:21:50 +0100'>2016-11-25</span></td><td><a href='/guix/commit/gnu/packages/upnp.scm?id=cdbdaf7b6b2c232bca5f2e84af3dca097a3f5ce6'>gnu: miniupnpc: Use ‘modify-phases’ syntax.</a><span class='msg-avail'>...</span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2016-11-25 02:21:50 +0100'>2016-11-25</span></td><td><a href='/guix/commit/gnu/packages/upnp.scm?id=2d3b0203f40ac70b2e6fdb734605515b0a4cac24'>gnu: miniupnpc: Update to 2.0.</a><span class='msg-avail'>...</span></td><td>Tobias Geerinckx-Rice</td></tr>