configure.ac: Improve icc_version guessing for non-icc environment

icc_version was wrongly defined as "__ICC" on non-icc C compiler, which
caused a warning:

```
./configure: line 8211: test: __ICC: integer expression expected
```

This change adds a sed commend to delete "__ICC".

configure.ac

@@ -239,7 +239,7 @@ AS_IF([test "$GCC" = yes], [
     gcc_minor=`echo =__GNUC_MINOR__ | $CC -E -xc - | sed '/^=/!d;s///'`
     test -n "$gcc_major" || gcc_major=0
     test -n "$gcc_minor" || gcc_minor=0
-    icc_version=`echo =__ICC | $CC -E -xc - | sed '/^=/!d;s///'`
+    icc_version=`echo =__ICC | $CC -E -xc - | sed '/^=/!d;s///;/^__ICC/d'`
     test -n "$icc_version" || icc_version=0
     # RUBY_APPEND_OPTIONS(XCFLAGS, ["-include ruby/config.h" "-include ruby/missing.h"])
 ], [