Log Message:
Update to 2.68, requested by adam for pkg-config-0.25.

* Noteworthy changes in release 2.68 (2010-09-22) [stable]
  Released by Eric Blake, based on git versions 2.67.*.

** AC_MSG_ERROR (and AS_ERROR) can once again be followed immediately by
   `dnl'.  Regression introduced in 2.66.

** AC_INIT again allows URLs with '?' for its BUG-REPORT argument.
   Regression introduced in 2.66.

** AC_REPLACE_FUNCS again allows a non-literal argument, such as a shell
   variable that expands to a list of functions to check.  Regression
   introduced in 2.66.

** AT_BANNER() with empty argument will cause visual separation from previous
   test category.

** The macros AC_PREPROC_IFELSE, AC_COMPILE_IFELSE, AC_LINK_IFELSE, and
   AC_RUN_IFELSE now warn if the first argument failed to use
   AC_LANG_SOURCE or AC_LANG_PROGRAM to generate the conftest file
   contents.  A new macro AC_LANG_DEFINES_PROVIDED exists if you have
   a compelling reason why you cannot use AC_LANG_SOURCE but must
   avoid the warning.

** The macro m4_define_default is now documented.

** Symlinked config.cache files are supported; configure now tries to
   update non-symlinked cache files atomically, so that concurrent configure
   runs do not leave behind broken cache files.  It is still unspecified
   which subset or union of results is cached though.

** Autotest testsuites should not contain long text lines any more, and be
   portable even when very many test groups are used.

** AT_CHECK semantics with respect to the Autotest variable $at_status and
   shell execution environment of the arguments are documented now.

** AC_FC_LIBRARY_LDFLAGS now tolerates output from newer gfortran.

** Newly obsolete macros
   The following macros have been marked obsolete.  New programs
   should use the corresponding Gnulib modules.  Gnulib not only
   detects a larger set of portability problems with these functions,
   but also provides complete workarounds.

     AC_FUNC_ERROR_AT_LINE  AC_FUNC_LSTAT_FOLLOWS_SLASHED_SYMLINK
     AC_FUNC_MKTIME  AC_FUNC_STRTOD