Fix for CC being sometimes a path and sometimes a file on PATH (#1724)

Possible fix for #1713

Co-authored-by: Jon Ross-Perkins <jperkins@google.com>

bazel/cc_toolchains/clang_configuration.bzl

@@ -26,18 +26,19 @@ def _detect_system_clang(repository_ctx):
     # If the user provides an explicit `CC` environment variable, use that as
     # the compiler. This should point at the `clang` executable to use.
     cc = repository_ctx.os.environ.get("CC")
+    cc_path = None
     if cc:
-        version_output = _run(repository_ctx, [cc, "--version"]).stdout
-        if not "clang" in version_output:
-            fail("The `CC` environment variable is not a Clang compiler.")
-        return repository_ctx.path(cc)
-
-    # Try looking on the path. We only check for the normal name.
-    system_clang = repository_ctx.which("clang")
-    if not system_clang:
-        fail("Unable to find a `clang` executable on the system path.")
-
-    return system_clang
+        cc_path = repository_ctx.path(cc)
+        if not cc_path.exists:
+            cc_path = repository_ctx.which(cc)
+    if not cc_path:
+        cc_path = repository_ctx.which("clang")
+    if not cc_path:
+        fail("Cannot find clang or CC (%s); either correct your path or set the CC environment variable" % cc)
+    version_output = _run(repository_ctx, [cc_path, "--version"]).stdout
+    if "clang" not in version_output:
+        fail("Searching for clang or CC (%s), and found (%s), which is not a Clang compiler" % (cc, cc_path))
+    return cc_path
 
 def _compute_clang_resource_dir(repository_ctx, clang):
     """Runs the `clang` binary to get its resource dir."""