configure.ac: fix the guess of icc_version

The icc_version guessing was accidentally disabled because of
61885c9b7ca8ccdaf53d7c365fbb86bad3294d88; `AC_PROG_CC_C99` changes
CC like "icc -std=c99", and `AS_CASE(["x$CC"], [xicc],` does not match.

The variable `icc_version` is eventually defined, so the `AS_CASE` is
not needed.  This change removes the `AS_CASE`.

configure.ac

@@ -239,9 +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 "$gcc_minor" || gcc_minor=0
-    AS_CASE(["x$CC"], [xicc], [
+    icc_version=`echo =__ICC | $CC -E -xc - | sed '/^=/!d;s///'`
-      icc_version=`echo =__ICC | $CC -E -xc - | sed '/^=/!d;s///'`
-    ])
     test -n "$icc_version" || icc_version=0
     # RUBY_APPEND_OPTIONS(XCFLAGS, ["-include ruby/config.h" "-include ruby/missing.h"])
 ], [