build: Fix warnings emitted by autoconf-2.69d.

build: Fix warnings emitted by autoconf-2.69d.

* gettext-tools/configure.ac: Use AC_LINK_IFELSE instead of AC_TRY_LINK. Don't
invoke AC_FUNC_VFORK.
* gettext-tools/m4/exported.m4 (gt_GLOBAL_SYMBOL_PIPE): Require LT_PATH_NM
instead of AC_PROG_NM.
* gnulib-local/m4/libxml.m4 (gl_LIBXML): Use AC_LINK_IFELSE instead of
AC_TRY_LINK. Use AC_PREPROC_IFELSE instead of AC_TRY_CPP.
* libtextstyle/configure.ac: Use AC_CONFIG_HEADERS instead of AM_CONFIG_HEADER.
* libtextstyle/gnulib-local/m4/libcroco.m4 (gl_LIBCROCO): Use AC_LINK_IFELSE
instead of AC_TRY_LINK.
* libtextstyle/gnulib-local/m4/libglib.m4 (gl_LIBGLIB): Use AC_LINK_IFELSE
instead of AC_TRY_LINK. Require AC_USE_SYSTEM_EXTENSIONS instead of AC_GNU_SOURCE.
* libtextstyle/gnulib-local/m4/termcap.m4 (gl_TERMCAP_BODY): Use AC_LINK_IFELSE
instead of AC_TRY_LINK.
* libtextstyle/gnulib-local/m4/terminfo.m4 (gl_TERMINFO_BODY): Use
AC_LINK_IFELSE instead of AC_TRY_LINK.
* m4/woe32-dll.m4 (gl_WOE32_DLL): Use AC_LINK_IFELSE instead of AC_TRY_LINK.