validate-clang-check: fix some issues.

Not all shells support [[ ]] compound commands; it's not in the most
recent Single UNIX Specification I could see, and the
ubuntu-clang-other-tests job is reporting

    tools/validate-clang-check.sh: 18: [[: not found

Don't use [[ ]].

In addition, if you change extcap/etl.c, it tries to run clang-check on
it, but that file builds, and is only built, on Windows, so clang-check
fails dismally on UN*Xes.  Omit it for now.

tools/validate-clang-check.sh

@@ -14,8 +14,17 @@ COMMIT_FILES=$( git diff-index --cached --name-status HEAD^ | grep -v "^D" | cut
 
 for FILE in $COMMIT_FILES; do
     # Skip some special cases
+    FILE_BASENAME="$(basename file)"
     # iLBC: the file is not even compiled when ilbc is not installed
-    [[ $FILE =~ /iLBC/ ]] && continue
+    if test "$FILE_BASENAME" = "iLBCdecode.c"
+    then
+        continue
+    fi
+    # extcap/etl.c: that compiles, and is compiled, only on Windows
+    if test "$FILE_BASENAME" = "etl.c"
+    then
+        continue
+    fi
 
     clang-check ../$FILE
     clang-check -analyze ../$FILE