--- a/tools/cpp/unix_cc_configure.bzl
+++ b/tools/cpp/unix_cc_configure.bzl
@@ -145,11 +145,18 @@ def get_escaped_cxx_inc_directories(repository_ctx, cc, lang_flag, additional_fl
     else:
         inc_dirs = result.stderr[index1 + 1:index2].strip()
 
-    return [
+    default_inc_directories = [
         _prepare_include_path(repository_ctx, _cxx_inc_convert(p))
         for p in inc_dirs.split("\n")
     ]
 
+    env = repository_ctx.os.environ
+    if "SPACK_INCLUDE_DIRS" in env:
+        for path in env["SPACK_INCLUDE_DIRS"].split(":"):
+            default_inc_directories.append(path)
+
+    return default_inc_directories
+
 def _is_compiler_option_supported(repository_ctx, cc, option):
     """Checks that `option` is supported by the C compiler. Doesn't %-escape the option."""
     result = repository_ctx.execute([