Libtool now builds with the help of Autoconf-2.50!

Libtool now builds with the help of Autoconf-2.50!
* configure.ac (AC_PREREQ):  Require Autoconf-2.50 or newer.
(AC_PROG_CPP, AC_EXEEXT, AC_OBJEXT):  Removed.  Autoconf-2.50
doesn't have the AC_REQUIRE bug that required these to be called
explicitly from here.  Autoconf-2.50 uses this file instead of...
* configure.in: ...this, which is now deleted.
* cdemo/configure.ac, cdemo/configure.in: Ditto.
* demo/configure.ac, demo/configure.in: Ditto.
* depdemo/configure.ac, depdemo/configure.in: Ditto.
* libltdl/configure.ac, libltdl/configure.in: Ditto.
* mdemo/configure.ac, mdemo/configure.in: Ditto.
* pdemo/configure.ac, pdemo/configure.in: Ditto.
* libltdl/acconfig.h:  Deleted.
* Makefile.am (libtool): libtool now depends on configure.ac.
* libtool.m4 (_LT_AC_LOCK):  Use 2.50's AC_LANG_PUSH/AC_LANG_POP
instead of 2.13's AC_LANG_SAVE/AC_LANG_RESTORE.
(AC_LIBTOOL_LANG_C_CONFIG): Ditto.
(AC_LIBTOOL_LANG_CXX_CONFIG): Ditto.
* libtoolize.in:  Use $configure_ac, instead of hardcoding
configure.in.