gnu: clang: Allow 'clang' to link executables.

* gnu/packages/patches/clang-libc-search-path.patch: New file.
* gnu-system.am (dist_patch_DATA): Add it.
* gnu/packages/llvm.scm (clang-from-llvm)[source]: Use it.
  [inputs]: Add "gcc-lib".
  [arguments]. Add -DGCC_INSTALL_PREFIX and -DC_INCLUDE_DIRS to
  #:configure-flags.  Add #:phases argument.

gnu/packages/patches/clang-libc-search-path.patch

@@ -0,0 +1,19 @@
+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'.
+
+--- 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
+@@ -3085,6 +3085,10 @@ Linux::Linux(const Driver &D, const llvm
+ 
+   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 {