clang: Makes builds with clang less dependent on absolute file path

build/config/compiler/BUILD.gn

Index: build/config/compiler/BUILD.gn
diff --git a/build/config/compiler/BUILD.gn b/build/config/compiler/BUILD.gn
index 97eb6726ca36455f5b87284c80ecc8c85f8c875e..49d98c1f4ca53f4171bdd9a40867fa2d3504dc55 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -310,6 +310,14 @@ config("compiler") {
     cflags += [ "-fcolor-diagnostics" ]
   }
 
+  # Makes builds independent of absolute file path.
+  # Currently disabled for nacl since its toolchain is old and doesn't have this flag.
+  # TODO(zforman): Once nacl's toolchain is updated, remove check.
+  if (is_clang && !is_nacl) {
+    absolute_path = rebase_path("../../..")

[Zachary Forman, 2016/01/05 03:21:29]

Is there a GN builtin or otherwise that allows us to get the root source
directory? I would use "//", but this has the unfortunate effect of giving a
rebased path that ends with a '/'. As some symbols include only the directory
name (no trailing slash), having a trailing slash in the prefix map misses some
cases.

[Nico, 2016/01/05 12:22:48]

^ brettw

[brettw, 2016/01/05 19:10:11]

I think you can do:
  rebase_path("//.")
The thing happening here is that GN tries to match whether the input ends in a
slash or not so that concatenating paths works the same before and after
rebasing, but for the root dir this is ambiguous.

[Zachary Forman, 2016/01/06 23:20:49]

This worked - thanks!

+    cflags += [ "-fdebug-prefix-map=$absolute_path=." ]
+  }
+
   # C++11 compiler flags setup.
   # ---------------------------
   if (is_linux || is_android || (is_nacl && is_clang)) {