Autoconf redefines the M4 builtin macro 'm4_undefine' in such a way that

it fails if the macro that is being undefined is not already defined. To
make this work under all cases and be sure that at a certain point some
specific macro isn't defined we must use the following style in configure:

m4_ifdef([macro], [m4_undefine([macro])])

ares/configure.ac

@@ -64,11 +64,15 @@ AC_HELP_STRING([--disable-debug],[Disable debug options]),
 )
 
 dnl skip libtool C++ and Fortran compiler checks
+m4_ifdef([AC_PROG_CXX], [m4_undefine([AC_PROG_CXX])])
 m4_defun([AC_PROG_CXX],[])
+m4_ifdef([AC_PROG_F77], [m4_undefine([AC_PROG_F77])])
 m4_defun([AC_PROG_F77],[])
 
 dnl skip libtool C++ and Fortran linker checks
+m4_ifdef([AC_LIBTOOL_CXX], [m4_undefine([AC_LIBTOOL_CXX])])
 m4_defun([AC_LIBTOOL_CXX],[])
+m4_ifdef([AC_LIBTOOL_F77], [m4_undefine([AC_LIBTOOL_F77])])
 m4_defun([AC_LIBTOOL_F77],[])
 
 dnl libtool setup

configure.ac

@@ -116,11 +116,15 @@ dnl support building of Windows DLLs
 AC_LIBTOOL_WIN32_DLL
 
 dnl skip libtool C++ and Fortran compiler checks
+m4_ifdef([AC_PROG_CXX], [m4_undefine([AC_PROG_CXX])])
 m4_defun([AC_PROG_CXX],[])
+m4_ifdef([AC_PROG_F77], [m4_undefine([AC_PROG_F77])])
 m4_defun([AC_PROG_F77],[])
 
 dnl skip libtool C++ and Fortran linker checks
+m4_ifdef([AC_LIBTOOL_CXX], [m4_undefine([AC_LIBTOOL_CXX])])
 m4_defun([AC_LIBTOOL_CXX],[])
+m4_ifdef([AC_LIBTOOL_F77], [m4_undefine([AC_LIBTOOL_F77])])
 m4_defun([AC_LIBTOOL_F77],[])
 
 dnl libtool setup