ci-env: build_and_test fails on centos6 and centos6_roundtrip

[hirooih]

The version of ctags: current HEAD

Since I merged #3054, build_and_test fails on centos6 and centos6_roundtrip.

...
m4/00gnulib.m4:78: error: m4_defn: undefined macro: _AC_CHECK_DECL_BODY
m4/00gnulib.m4:78: the top level
...

It failed on the following code in m4/00gnulib.m4 added by #3054;

dnl Redefine _AC_CHECK_DECL_BODY so that it references ac_compile_for_check_decl
dnl instead of ac_compile.  If, for whatever reason, the override of AC_PROG_CC
dnl in zzgnulib.m4 is inactive, use the original ac_compile.
m4_define([_AC_CHECK_DECL_BODY],
[  ac_save_ac_compile="$ac_compile"
  if test -n "$ac_compile_for_check_decl"; then
    ac_compile="$ac_compile_for_check_decl"
  fi]
m4_defn([_AC_CHECK_DECL_BODY])[  ac_compile="$ac_save_ac_compile"
])

According to this article of the Gnulib ML, the code above should fail on RHEL 6 / CentOS 6 which are shipped with autoconf 2.63.
This was known issue.

Strange thing to me is it passed during debugging #3054 like this.
I don't see any commits related to this after the test passed.

Shall we remove centos6 and centos6_roundtrip test, or are there any better way?