describe glibc version bit more accurate.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@17858 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

configure.in

@@ -704,7 +704,7 @@ AC_FUNC_MEMCMP
 # http://sources.redhat.com/ml/libc-hacker/2005-08/msg00008.html
 # Debian GNU/Linux Etch's libc6.1 2.3.6.ds1-13etch5 has this problem.
 # Debian GNU/Linux Lenny's libc6.1 2.7-10 has no problem.
-AC_CACHE_CHECK(for broken erfc of glibc-2.3 on IA64, rb_broken_glibc_ia64_erfc,
+AC_CACHE_CHECK(for broken erfc of glibc-2.3.6 on IA64, rb_broken_glibc_ia64_erfc,
   [AC_TRY_RUN([
 #include <math.h>
 int